In 1962, he founded the planning consultancy Percy Johnson-Marshall & Associates, commissioned to masterplan the University of Edinburgh's Comprehensive Development Area in the 1960s.
Johnson-Marshall worked as a Senior Planner with London County Council from 1949 to 1959, overseeing several Comprehensive Development Areas, including Lansbury Estate.
A new department of Urban Design and Regional Planning was established in 1964, with Percy Johnson-Marshall as the first professor, within the School of the Built Environment headed by Sir Robert Matthew.
Within the UK, PJMA worked on redevelopment schemes in towns including Newcastle-upon-Tyne, Coleraine, Northern Ireland, Kilmarnock, Ayrshire, and Salford in Greater Manchester.
The firm acquired Glasgow practice McKeown Alexander in 2001, and Edinburgh architects Wheeler & Sproson in 2005, and now employs nearly 150 people in five offices across the UK.
National Life Stories conducted an oral history interview (C467/2) with Percy Johnson-Marshall in 1990 for its Architects Lives' collection held by the British Library.