Other big screen credits include a turn as a forensics professor in "Gorky Park" (1983) and featured roles in "Dirty Rotten Scoundrels" (1988), directed by fellow "Star Wars" veteran Frank Oz, and "Restoration" (1995). On US television, McDiarmid has appeared in several programs that have aired on PBS' "Mystery!", including 1992's "Masonic Mysteries", directed by Danny Boyle, and 1998's "Sacrifice" and "Touching Evil". He acted in Renny Rye's adaptation of Dennis Potter's "Karaoke" and reprised his role of Oliver Morse in the follow-up "Cold Lazarus" (both aired on Bravo in 1997). In 1999, he portrayed Jaggers in a BBC/PBS production of "Great Expectations".

Along with Jonathan Kent, Ian McDiarmid served as joint artistic director of the Almeida Theatre, one of the more successful and influential of London's playhouses. With many of their productions attracting big name performers such as Diana Rigg, Ralph Fiennes, Juliette Binoche and Kevin Spacey, and others transferring to both West End and Broadway theatres, the Almeida has served to further the influence of the stage and has brought works by playwrights like Pirandello and Racine to a public who may not have experienced them otherwise.

An exceptional stage actor and director, McDiarmid has performed in several productions for the theater since the mid-1980s including "The Saxon Shore", "Creditors", "Volpone", "Ivanov", and "The Jew of Malta". As a stage director, some of McDiarmid's credits include "The Possibilities", "The Rehearsal", "Hippolytus" and "A Hard Heart", all for the Almeida, and "Don Juan" at the Royal Exchange Theatre. Like many players, McDiarmid began his stage work in repertory, first with Glasgow's Citizen's Theatre, then in Manchester and later with the Royal Shakespeare Company, where he acted in "Measure for Measure", "Macbeth", "Henry V" and "The Danton Affair", among others.