Actor and theatre impresario. McDiarmid was born in Carnoustie and brought up in Dundee. He gained a degree from St. Andrews University before attending the Royal Scottish Academy of Music and Drama (Glasgow) and where he won a gold medal in 1968. He began his acting career in Repertory Theatre but, after 10 years, joined the Royal Shakespeare Company.
He appeared in the television series The Professionals (1979), but came to international prominence as Emperor Palpatine, who comes to a grisly end at the hands of Darth Vader in the final moments of the Star Wars epic Return of the Jedi (1983). He was to play a younger version of the same role in the The Phantom Menace (1999) and is contracted to appear again in two further sequels. Other screen roles have included Gorky Park (1983) and Dirty Rotten Scoundrels (1988) and Tim Burton's Sleepy Hollow (1999), with Great Expectations (1999) and Crime and Punishment (2002) for television.
In 1990, McDiarmid joined the Almeida Theatre as both actor and its Artistic Director and has promoted the reputation of that theatre to an important London venue. There, he has performed in plays including The Doctor's Dilemma, The Government Inspector, Hippolytos, the title role in Volpone, The School for Wives, Tartuffe, The Cenci and Lulu, which he has also directed. He has directed a number of plays and the opera Siren Song.
McDiarmid, along with his co-director Jonathan Kent, were gained the 'Theatrical Achievement of the Year' award in 1998. McDiarmid is one of London theatre's most influential figures.