Penson's father had been County Surveyor of Flintshire, responsible for a bridge at Overton-on-Dee which collapsed. Thomas Penson designed its replacement.
Penson was a pupil, along with Thomas Telford, of the architect and bridge designer Thomas Harrison. He relied on contractors to build his bridges, such as David Davies, who built Llandinam Bridge in 1846.
In February 1852, the Severn flooded, damaging a number of bridges for which Penson went on to design replacements.
Penson designed a two-span cast iron arch bridge at Caer Howell in 1858 to replace a timber structure destroyed by floods. Against his advice, a suspension bridge designed by James Dredge had been built in 1854, only to collapse four years later under the weight of three lime wagons, killing one man. He also designed a church at Newtown, Powys, and Montgomery jail.
Other structures credited to Thomas Penson include: