It was authorised under an Act of Parliament dated 20 August 1853, to operate from a junction with the Newport, Abergavenny & Hereford Railway, near Pontypool, to Coleford, Gloucestershire, with a branch to serve the gas works at Monmouth.

The four miles (6.4 km) from Little Mill Junction to Usk were opened on 2 June 1856, the line was worked by the Newport, Abergavenny and Hereford Railway.

The twelve miles (19 km) onwards to Monmouth Troy opened on 12 October 1857 when the company worked its line with engines hired from the Newport, Abergavenny and Hereford Railway.

From 1940 to 1946, the Little Mill Junction to Glascoed section of the line further west was heavily used to carry shift workers from the Eastern Valleys to the Royal Ordnance Factories' munitions production site's three platform station.

Otherwise, passenger traffic was always light, although some attempts were made to revive ailing fortunes with the addition of an evening train from Monmouth and the opening of Elms Bridge Halt in the 1930s.