Ptolemy qualified as an architect at Edinburgh University before winning an ICOMOS scholarship, after which he worked on various Grade I listed buildings including Stowe and Chastleton House. In 1993 he became a SPAB Lethaby Scholar, with a specialism in Sir John Soanes' country houses. Ptolemy joined Richard Griffiths Architects in 1994, working on Southwark Cathedral, before establishing his own architectural practice. In 2012 he was appointed Surveyor of the Fabric at Westminster Abbey and has served on the London Advising Panel of English Heritage, the Architectural Panel of The National Trust and is on the committee of SAVE Britain's Heritage.