Law Lord. Born in Falkland (Fife), the eldest son of the town's Provost, Deas was educated in Falkland, Milnathort and Perth, before joining solicitors' offices in Perth and then Cupar. He then studied law at the University of Edinburgh before being called to the Bar in 1828. He served Advocate Depute (1840-50), Sheriff of Ross and Cromarty (1850-1), Solicitor General (1851-2), and was created a Lord of the Court of session, as Lord Deas, in 1853 and a Lord Commissioner of Justiciary in 1854. He was knighted in 1858 and awarded an honorary degree by the University of Edinburgh in 1884.
He had a home at Pittendreich House (Midlothian) and also owned an estate, Hartwoodhill, in North Lanarkshire. Deas died at his house in Heriot Row, Edinburgh, and is buried in Warriston Cemetery.