The Aberystwyth and Welsh Coast Railway authorized the bridge in 1861 and began construction in 1864. Spanning the broad Mawddach estuary between Barmouth to the north and Morfa Mawddach near Arthog to the south, the bridge sits within both a Site of Special Scientific Interest and Snowdonia National Park. Horse-drawn carriages began crossing on 3 June 1867, and steam services followed on 10 October. To allow tall sailing ships to pass, the original design incorporated a drawbridge section, later replaced by a swing bridge between 1899 and 1902. The timber section alone stretches 699 meters, built atop a gravel bed covered by shifting sand, with water flowing through the northern channel at up to nine knots.

The bridge has survived threats that would have finished lesser structures. In 1946, a live naval mine washed past one of the pillars during a storm but failed to detonate. By 1980, marine woodworm -- Teredo navalis, the shipworm that has plagued timber structures since the age of sail -- had eaten into 69 of the supporting pillars at riverbed level. British Rail divers estimated repairs at 2.5 million pounds. The viaduct closed to rail traffic on 13 October 1980, and there was serious talk of permanent demolition. Gwynedd County Council fought to save it, arguing that 40 percent of local tourism was rail-dependent. The bridge reopened in April 1986 after extensive repairs, though with weight restrictions and a ban on locomotive-hauled trains. In October 2016, a fire on the structure closed it for a week, prompting fresh calls for urgent renovation.

Pedestrians and cyclists share the bridge via a footpath on its eastern side, part of National Cycle Route 8 linking Cardiff to Holyhead. Tolls were collected for foot and cycle traffic until 2013, when the collectors simply left and were not replaced. Gwynedd Council, struggling to cover the 30,800-pound annual maintenance contribution to Network Rail, considered closing the footpath entirely. A petition against closure attracted 20,000 signatures in a single week, and in February 2016 the council backed down. In late 2017, someone had the inspired idea of introducing an "honesty toll" of one pound for adults and 50 pence for children, complete with a troll mascot. The old toll house was rebranded as a "troll house," adding a touch of whimsy to a structure that has spent most of its existence fighting for survival.

The bridge's most recent and most expensive restoration began in 2020, when Network Rail committed 30 million pounds to a multi-year project. The work required three separate seasonal closures, the last running from autumn through December 2023. On 2 December 2023, the track reopened, and a formal ceremony the following Friday drew the Wales Office minister, a Senedd member, the mayor of Barmouth, and representatives of Network Rail and its engineering contractors. The swing-bridge mechanisms were not restored during this refurbishment, ending the bridge's ability to open for tall ships. But the timber trestles were renewed, the ironwork strengthened, and the line secured for the foreseeable future. The viaduct had celebrated its 150th anniversary in October 2017 with fireworks and charter trains. At 156 years old and counting, it remains the most improbable piece of railway engineering in Wales.