The firm, Quinlan Terry Architects LLP, continues the architectural style of the practice started by Raymond Erith in 1928, and specialises in high quality traditional building, mostly in classical idioms.

A book about the firm's work, written by David Watkin, entitled Radical Classicism: The Architecture of Quinlan Terry (New York: Rizzoli International Publications), was published in 2006.

The first work by Raymond Erith in which Quinlan Terry had a major role was the new house, Kings Waldenbury, Hertfordshire, completed for the Pilkington family in 1971, when new building in a classical manner was deeply unfashionable with the architectural establishment (though it was more popular with the general public).

This is a radical extension of a 19th-century Roman Catholic Gothic revival church is in the English Baroque manner owing much to James Gibbs and Thomas Archer and makes little or no attempt to be in keeping with the older building.

During the 1980s he was appointed by Margaret Thatcher, then Prime Minister, to renovate the interiors of 10 Downing Street, restored 40 years previously by Raymond Erith, Terry's teacher, after war damage.

In Gloucestershire, he designed Waverton House, where he used the style made popular by Matthew Brettingham in the late 18th century, featuring a central staircase lit from above, surrounded by rooms on both floors.

In the mid-1990s, Terry designed the restoration of St Helen's Bishopsgate, controversially turning the orientation of the medieval church through 90 degrees, moving or removing some fittings, and reworking its previous Tractarian Anglican layout into a Georgian stripped-back meeting house plan informed by the precepts of Reformation theology, in tune with its current firmly evangelical congregation.

[5] Terry designed the external envelope of New Margaret Thatcher Infirmary at the Royal Hospital Chelsea, with Steffian Bradley Architects[6] as the lead consultant and planners for the building; a new Georgian Theatre for Downing College Cambridge; new offices, retail and residential development at 264–267 Tottenham Court Road, London; offices and retail at 22 Baker Street, London; and Queen Mother Square, Poundbury; and mixed use development Richmond Riverside.