Selby began his career on the stage, in repertory, and later established himself as a member of the Royal Shakespeare Company from 1963 until 1976, often as authoritative noblemen and bishops. Outside of the RSC, he originated the role of Baron von Swieten in Amadeus in 1979 (reprising it on radio in 1983). Concurrent with his Royal Shakespeare work, Selby appeared in the films A Midsummer Night's Dream and the 1971 Macbeth (as Duncan). On television, in and out of period, he was often cast as bankers, judges, or other men of substance, playing scheming banker Nicholas Warleggan in the 1975 Poldark series, Lord Billsborough in the 1990 miniseries House of Cards, and Sir Walter Raleigh in the final installment of Elizabeth R (starring Glenda Jackson).