Hertfordshire on Wikipedia.

This website has a comprehensive collection of Hertfordshire churches.
 

Abbots Langley.
Albury, St. Mary the Virgin. © Bill McKenzie.
Aldbury, St. John the Baptist. Another view showing the recent extension, used as a meeting room, and called Chapter House. SP 963 124. Both © Richard Pykett (2018). Link. Grade I listed.
Allen's Green, the former St. Anne, which now serves as the village hall. It's dated here to 1863 (as a Mission Church), with closure no later than 2012. Another view. TL 4549 1682. Both © Karel Kuča (2019). At one time there was also a Congregational Chapel, north of the church, at TL 4554 1691. It pre-dated a map of 1879, and appears to have gone out of use in the first two decades of the 20th century. The building on the site today can be seen in a Streetview from 2010 - whether anything remains of the chapel is unknown.
Anstey, St. George. TL 4043 3289. From an old postcard in Reg Dosell's Collection. Two modern views - 1, 2, two of the interior - 1, 2, two windows - 1, 2, and the superb font, all © Chris Stafford (2013). Link. Grade I listed. Old maps show a Primitive Methodist Chapel at TL 4075 3331. It's not apparent from Streetview whether it still survives or not, but the building glimpsed here in the background, its roof showing above the hedge, must stand on or near the site of the chapel. Another Chapel is marked on the 1" map of 1956, a short distance south of the P.M. Chapel, at TL 4069 3324. Very well hidden by trees and hedges, a 2009 Streetview offers a very restricted view. The village website offers a photo.
Ardeley, St. Lawrence. TL 318 272. © Les Needham.
Ashwell, St. Mary the Virgin. TL 2670 3978. © Bill McKenzie. Two more views - 1, 2, and a graffito probably showing Old St. Paul's Cathedral in London, all © Christopher Skottowe (1961 and 1963). An old postcard view, from Reg Dosell's Collection. Link. Grade I listed. The U.R.C. on High Street, Silver Street and Kingsland Way was formerly Congregational. TL 2691 3967. © Les Needham. Link. Zoar Strict Baptist Chapel on Gardiner's Lane, and its interior. It's dated here to 1854. TL 2664 3972. Both © Gerard Charmley (2021). Old maps indicate a Primitive Methodist Chapel between High Street and Back Street, at TL 2645 3951. It can be seen in the background (above the red car) in a Streetview from 2016. More photos are available here, where it's dated to 1861, and described as now being in residential use.
Aspenden, St. Mary. TL 353 284. © Les Needham. Link.
Ayot St. Lawrence, St. Lawrence Old Church. © Bill McKenzie.
Ayot St. Peter, St. Peter. © Bill McKenzie.

Babbs Green, former church. © Bill McKenzie. Janet Gimber advises that this was Wesleyan Methodist.
Baldock.
Barkway, St. Mary Magdalene. TL 383 356. Link. Grade I listed - link. Barkway Chapel (Congregational). TL 385 359. Both © Les Needham (2011).
Bayford, St. Mary. © Bill McKenzie.
Bedmond, Church of the Ascension. © Les Needham. Link.
Bengeo, Hertford - see Hertford.
Berkhamsted.
Bishop's Stortford.
Bourne End - see Broadway, below.
Bovingdon, St. Lawrence. © Derek Collier. Link.
Bramfield, St. Andrew. TL 328 125. © Thomas Curtis. Link.
Brent Pelham, St. Mary the Virgin. TL 4336 3081. © Graeme Wall. A set of stocks in the churchyard, two interior views - 1, 2, the stone pulpit, and the font, all © Chris Stafford (2013). Link. Grade II listed.
Broadway, St. John the Evangelist (interior). Previously in the Unknown section, and solved by Janet Gimber, here's the original entry - "The first, a rather grand interior, is of St. John's Church, Broadway. There doesn't seem to be a St. John's in either the Worcestershire or Somerset Broadway. There's also a Broadway in Co. Wexford. There are no other clues on the postcard". TL 016 065. Postcard  from Judy Flynn's Collection. Grade II listed - link. Link - which says the church is at Bourne End.
Broxbourne, St. Augustine. From an old postcard (franked 1921?) in Steve Bulman's Collection.  U.R.C. on Mill Lane.  © Janet Gimber (2018).
Buckland, St. Andrew. Now in the care of the Churches Conservation Trust. TL 3584 3386. © Les Needham (2011). Two more views - 1, 2, two of the interior - 1, 2, a window detail, and the font, all © Chris Stafford (2013). Link. Grade II* listed.
Buntingford, the shell of St. Bartholomew at Layston, which was the original parish church. The cemetery remains in use. TL 369 301. St. Peter is the present parish church. TL 363 293. Link. St. Richard (R.C.). TL 363 292. U.R.C. TL 361 294. All © Les Needham.
Bushey and Bushey Heath.
Bushey Heath, St. Peter. Link to external site.

Chipperfield, St. Paul. TL 043 015. Link. Baptist Church. TL 041 018. Both © Chalmers Cursley.
Chorleywood.
Clothall, St. Mary the Virgin. TL 271 320. © Les Needham (2011). Link1. Link2. Grade II* listed - link.
Codicote, St. Giles. TL 219 188. © Thomas Curtis. Link.
Colliers End, St. Mary. Interior (taken through a window). Both © Chris Stafford (2013). Link.
Cottered, St. John the Baptist. TL 318 292. © Les Needham. Link.
Croxley Green.

Damask Green, Chapel (1861). TL 259 300. © Les Needham (2011).
Datchworth, St. Andrew. © Bill McKenzie.

Flamstead End, St. Leonard. © Bill McKenzie.
Flaunden, St. Mary Magdalene. TL 013 008. © Chalmers Cursley. The old church was demolished in 1843/4. From an old engraving in Colin Waters' Collection. Former chapel, now a private house. TL 016 010. © Chalmers Cursley. Janet Gimber advises that this was "Baptist Chapel (General)", and later just Baptist. One old map marks it as Flaunden United Chapel (Baptist).
Furneux Pelham, St. Mary the Virgin. TL 4316 2795. © Graeme Wall. Another view, © Karel Kuča (2019). Link. Grade II* listed.

Great Amwell, St. John the Baptist. © Bill McKenzie. An old postcard view (card franked 1909), from Reg Dosell's Collection. Link.
Great Gaddesden, St. John the Baptist. © Bill McKenzie. Another view. © Derek Collier.
Great Hormead, St. Nicholas. Two views of the interior - 1, 2, a window, and the font. The church has some excellent stone-carved heads - 1, 2. TL 3999 2957. All © Chris Stafford (2013). Two further views - 1, 2, both © Karel Kuča (2019). Link. Grade II* listed. Older maps show a Congregational Chapel on Horseshoe Lane at TL 4023 2988. It pre-dates a map of 1877, and the last map I can find which labels it is from 1960. Whether the building on the site today is a conversion or new build on the same site is so far unknown. © Karel Kuča (2019).
Great Wymondley, St. Mary. TL 215 285. © Thomas Curtis. Link.

Hadham Cross, Congregational Church. TL 427 186. © Chalmers Cursley. Another view. © Bill McKenzie.
Harpenden.
Hatfield, St. Etheldreda. Another view. TL 235 085. Both © Thomas Curtis. Link.
Hemel Hempstead.
Hertford.
Hertingfordbury, St. Mary. © Bill McKenzie.
Hexton, St. Faith. Another view. Both © Bill McKenzie.
High Cross, St. John the Evangelist. Another view. TL 3639 1867. Both © Chris Stafford (2013). Its grade II listing dates it to 1846, with tower added in 1906. Link has numerous photos.
High Leigh, The Chapel. Previously in the Unknown section, The Chapel, High Leigh is from a postcard in Judy Flynn's Collection. Janet Gimber and Brian Curtis have both confirmed that this is the High Leigh near Hoddesdon in Hertfordshire, to be precise, at the High Leigh Conference Centre. Using Bing maps to look at the postcode EN11 8SH, and zooming in onto the cross on the Ordnance Survey map option, just above the text "High Leigh", then switch to the birds-eye view, you can see the building apparently unchanged from when the postcard photo was taken.
Hinxworth, St. Nicholas. © Bill McKenzie.
Hunsdon, St. Dunstan. TL 418 127. © Chalmers Cursley. Link.
Hoddesdon.

Ickleford, St. Kathryn. © Bill McKenzie.

Kelshall, St. Faith. © Alan Craxford. An old postcard view from Reg Dosell's Collection. Link.
Kensworth, St. Mary the Virgin. © Bill McKenzie.
Kimpton, St. Peter & St. Paul. © Bill McKenzie.
Kings Langley.
King's Walden, St. Mary. © Bill McKenzie. Link.
Knebworth, St. Martin. © Bill McKenzie. St. Mary, at Knebworth House. TL 231 210. © Thomas Curtis. Link.

Langford, St. Andrew, as seen by Streetview in 2020. TL 1856 4140. Link. Grade I listed. The former Salvation Army Hall on High Street. TL 1862 4089. © Gerard Charmley (2021). The Methodist Church on High Street is marked on old maps as Wesleyan. TL 1854 4063. It can be seen on a Streetview from 2009. Link.
Langleybury, St. Paul. TL 080 006. © Les Needham. Link.
Lemsford, St. John the Evangelist. © Bill McKenzie. An old postcard view, from in Reg Dosell's Collection. Link.
Letchworth.
Leverstock Green, Holy Trinity. TL 085 065. © Les Needham. Link.
Lilley, St. Peter. Its grade II* listing dates it to a re-build of 1870-2, incorporating some of the medieval fabric. TL 1183 2637. From an old postcard (franked 1908) in Judy Flynn's collection. Streetview provides some alternative views - 1 (2010), 2 (2010), 3 (2020). For details of two listed monuments in the churchyard, see here. A converted former Wesleyan Methodist Chapel stands off West Street at TL 1172 2660. It was (just) glimpsed by Streetview in 2014 - the white building behind the tree, and here's the footpath which accessed it, from Streetview in 2010. Some photos of the former chapel are available here. This photo (© Bill McKenzie) previously listed as being St. Peter clearly isn't. Judy thinks it could well be a Hertfordshire church, it having the distinctive Hertfordshire spike. Can you identify it?
Little Amwell, Holy Trinity. © Bill McKenzie.
Little Berkhampstead, St. Andrew. © Bill McKenzie.
Little Gaddesden, St. Peter and St. Paul. Further views - 1, 2, 3, 4, 5, an interior view, and a window. SP 997 138. All © Dave Westrap (2010). Link1. Link2.
Little Hadham, St. Cecilia. © Bill McKenzie.
Little Hormead, St. Mary, which is now cared for by the Churches Conservation Trust. Another view, and a blocked doorway. All © Karel Kuča (2019). Link. Grade I listed.
Long Marston, All Saints. SP 895 158. © Bill McKenzie. Another view.  The church dates from the 1880's, and was built to replace a medieval chapel of ease, of which only the 15th century tower survives. SP 894 156. Both © Les Needham. The former Baptist Chapel, now in residential use, dates from 1863. SP 898 155. © Les Needham. Former Wesleyan Chapel (1864), now also a private residence. SP 898 157. © Les Needham.

Markyate, St. John the Baptist. © Bill McKenzie.
Meesden, St. Mary, undergoing works in 2013. It has a fine brick porch. Two interior views - 1, 2, a monument, medieval tiles, and the font. TL 4389 3256. All © Chris Stafford (2013). Link. Grade II* listed.
Monken Hadley, St. Mary the Virgin. © Bill McKenzie.
Much Hadham, St. Andrews Holy Cross (Congregational and R.C.). TL 430 196. © Bill McKenzie. Another view. © Chalmers Cursley. Link (R.C.).

Nasty, the Gospel Hall. TL 3596 2444. © Karel Kuča (2019).
Nettleden, Church of St. Lawrence. © Bill McKenzie.
Newgate Street, St. Mary Ponsbourne. © Bill McKenzie.
North Mymms, St. Mary. © Eddie Murray. Link.
Northaw, dedicated to St. John the Evangelist. © Bill McKenzie. An old postcard view, from Reg Dosell's Collection.
Northchurch, St. Mary. SP 974 088. Baptist Church. SP 975 086. Link. Both © Les Needham.
Norton - see Letchworth.

Pirton, St. Mary the Virgin. © Bill McKenzie. Despite being labelled as St. Mary Magdalene, Great Offley, this old postcard in Reg Dosell's collection is clearly Pirton. Thanks to Janet Gimber for unravelling this little mystery. Link.
Potten End, Holy Trinity. TL 017 089. © Les Needham.
Puckeridge, St. Thomas of Canterbury (R.C.). Old maps suggest it was built in the 1960's or early 1970's. TL 3853 2310. © Karel Kuča (2019). Link. Just a short distance to the south, and on the opposite side of the road is a (former?) Congregational Chapel, seen by Streetview in 2016. It pre-dates a map of 1878-9. TL 3846 2309.
Puttenham, St. Mary. SP 885 149. © Les Needham. Link.

Radwell, All Saints. TL 232 359. © Les Needham (2010).
Redbourn, St. Mary. Previously in the "Unknown" section. this is another ID by Simon Davies. This glass negative, from around 1900, was taken by a great-uncle of Ian Mead. Link.
Reed, St. Mary. TL 361 357. © Les Needham (2011).
Rickmansworth.
Ridge, St. Margaret. TL 214 005. © Thomas Curtis. Link.
Royston, St. John the Baptist. Interior view. Both © Alan Craxford. Link. Grade I listed - link. St. Thomas of Canterbury and the English Martyrs (R.C., opened 1919). From an old postcard in Judy Flynn's Collection. Link.
Rushden, St. Mary. TL 306 318. From an old postcard in Reg Dosell's Collection. A modern view. © Les Needham (2011). Link.

St. Albans.
St. Ippolitts, St. Ippolitts. The tower. TL 198 272. Both © Bill McKenzie. Another view. © Thomas Curtis. Link.
St. Paul's Walden, dedicated to All Saints. © Bill McKenzie.
Sandon, All Saints. TL 322 346. © Les Needham (2011). Link. Grade I listed - link.
Sandridge, St. Leonard. TL 172 106. © Thomas Curtis. Link.
Sarratt, Church of the Holy Cross. TQ 038 984. © Derek Collier. Link.
Standon, St. Mary.  © Chris Emms (2010). Link. Grade I listed - link.
Stansted Abbotts, St. Andrew. © Bill McKenzie. An old postcard view, from Reg Dosell's Collection.
Stapleford, St. Mary (K). From an old postcard in Reg Dosell's Collection. Two modern views - 1, 2, and an interior, taken through a window, all © Chris Stafford (2013). Link.
Stevenage.
Stocking Pelham, St. Mary the Virgin (K). TL 4477 2928. © Graeme Wall. Another view, © Karel Kuča (2019). Interior view (taken through a window), © Chris Stafford (2013). Link. Grade II listed.

Tewin, St. Peter. TL 268 143. © Thomas Curtis. Link.
Therfield, St. Mary the Virgin. TL 3343 3709. © Alan Craxford. Link. Grade II listed. A churchyard monument is separately listed, also as grade II. Therfield Chapel (Congregational) on The Causeway. TL 3356 3728. © Alan Craxford. Link.
Throcking, Holy Trinity. TL 339 302. © Les Needham. Link.
Thundridge, St. Mary. Two interior views - 1, 2, and the font. TL 3587 1718. All © Chris Stafford (2013). Link. Grade I listed, wherein it's dated to 1853. The remains (essentially, the tower) of Thundridge Old Church stand about half a mile to the east, at TL 3683 1738. Dedicated to St. Mary and All Saints, it's grade II* listed. Link.
Tring.

Walkern, St. Mary the Virgin. TL 302 266. Link. U.R.C. TL 301 265. Both © Les Needham.
Wallington, St. Mary. TL 292 336. © Les Needham (2011). Grade II* listed - link.
Ware.
Warners End, St. Alban. TL 041 075. © Les Needham.
Waterford, St. Michael & All Angels. © Bill McKenzie. Another view, © Chris Stafford (2013).
Watford.
Watton-at-Stone, St. Mary & St. Andrew. © Bill McKenzie.
Welwyn, St. Mary the Virgin. © Bill McKenzie.
Welwyn Garden City.
Weston, Holy Trinity. TL 266 300. © Les Needham (2011). Link. Grade I listed - link.
Wheathampstead, St. Helen. © Bill McKenzie.
Widford, St. John the Baptist. TL 413 158. © Bill McKenzie. Another view. © Chalmers Cursley. And an old postcard view, from Reg Dosell's Collection.
Wigginton, St. Bartholomew. SP 940 103. Link. Baptist Chapel. SP 939 101. Both © Les Needham.
Willian, All Saints. Another view. Both © Christopher Skottowe. Link.
Wilstone, St. Cross. SP 904 140. © Les Needham. Link.
Wyddial, St. Giles. © Tim Tomlinson.