The present stone bridge was erected late in the 1200s. During the period 1988-1990 the Monmouth Flood Alleviation Scheme was effected in order to protect Monmouth from occasional flooding by the Monnow River and excavations revealed remains of the previous wooden bridge directly under the present stone one with a construction date of up to c.1180.

The gatehouse on the Monnow Bridge, Monnow Gate, was not an original feature of the bridge and it formed part of new town defences begun at the beginning of the 14th century. Other than their defensive uses, Monnow Gate and the other town gates were used regularly over more than five hundred years as the sites of toll collections. Tolls were authorized in the patent rolls of 1297 and 1315, and, in subsequent Monmouth charters, tolls were allowed for various specific purposes.

Few accounts survive of the gate's military use, although the bridge Bridge receives a brief mention in a letter from 1645 regarding a Royalist attempt to recover Monmouth. By the turn of the 18th century it was in need of maintenance and in 1705, rebuilding of the battlements as solid walls and raising the roof allowed the gate to be used as a house and a two-floored timber-framed extension now abutted the gate at its eastern corner. A part was also used, when required, as a lock-up or guardroom, requiring the domestic occupant to temporarily vacate the building.

The lean-to building was demolished in 1815 and a separate pedestrian passage was constructed in 1819. A second pedestrian passageway was constructed in 1845, the last significant structural change. Since the turn of the 20th century the ever-increasing volume of motorized traffic led to parallel increases in collisions and obstructions at the gate. In 2004 a new bridge was opened a few metres downstream of Monnow Bridge, and on the same day the old bridge became pedestrianized. Plans are being made to undertake a thorough conservation programme.