Callow's first role in the theatre was working for Sir Laurence Olivier – in the box office of the National Theatre. This offer of work came by return of post following a fan letter to Olivier. It was while watching actors rehearse that Simon realised that acting was something he wanted to do.

Simon made his stage debut in 1973, appearing in The Thrie Estates at the Assembly Hall Theatre, Edinburgh. In 1975 he acted in Martin Sherman's critically acclaimed Passing By for Gay Sweatshop theatre company, The Plumber’s progress in the West End and  Snoo Wilson's The Soul of the White Ant at the Bush Theatre. In 1977 he joined the Joint Stock Theatre; in 1978, he acted in Mary Barnes at the Royal Court, played the title roles in Titus Andronicus at the Bristol Old Vic and Arturo Ui at the Half Moon Theatre.

It was his critically acclaimed performance as Mozart in the original stage production of Peter Shaffer’s Amadeus at the Royal National Theatre in 1979 that brought Callow to greater prominence. It also led to his first film role, playing Schikaneder in Miloš Forman’s film of the play. Following this, he starred in several series of the Channel 4 sitcom Chance in a Million, as the eccentric Tom Chance, and appeared with Saeed Jaffrey in 1994 British television series Little Napoleons. In the same year, Callow played the much-loved character Gareth in the hit film Four Weddings and a Funeral.

Never known for his lack of energy or interests, Callow simultaneously pursued careers as a director and writer. His first book, written at the encouragement of the legendary literary agent Peggy Ramsay, Being An Actor (1984), was a critique of 'director dominated' theatre. It also contained autobiographical sections relating to his early career as an actor and marked one of the first times an actor had publicly come out about their homosexuality. At a time when subsidised theatre in the United Kingdom was under severe pressure from the Thatcher government, the work's original appearance caused a minor controversy. In 1988, Simon directed the West End premiere of Willy Russell’s Shirley Valentine, starring Pauline Collins. In 1992, he directed the play Shades by Sharman MacDonald and the musical My Fair Lady featuring costumes designed by Jasper Conran. These were followed by a stage version of the classic French film Les Enfants du Paradis for the RSC in 1995.

Simon’s lifelong passion for classical music has seen him directing opera productions and appearing alongside various orchestras around the world as well as fronting documentaries and writing about composers and their work. In 1996, he directed Cantabile in three musical pieces (Commuting, The Waiter's Revenge, Ricercare No. 4) composed by his friend Stephen Oliver. Ricercare No. 4 was commissioned by Callow especially for Cantabile.

One of Callow's best-known books is Love is Where it Falls, a poignant analysis of his eleven-year relationship with Peggy Ramsay. He has also written extensively about Charles Dickens, whom he has played in a one-man show, The Mystery of Charles Dickens by Peter Ackroyd, in the film Hans Christian Andersen: My Life as a Fairytale, and on television several times including An Audience with Charles Dickens (BBC, 1996) and in The Unquiet Dead, a 2005 episode of the BBC science-fiction series Doctor Who. He returned to Doctor Who for the 2011 season finale, again taking the role of Dickens.

Renowned for a series of one-man shows, which have toured the United Kingdom and internationally, these began with a West End production of The Importance of Being Oscar. They have included The Mystery of Charles Dickens by Peter Ackroyd, There Reigns Love, a performance of Shakespeare’s sonnets, Dr Marigold & Mr Chops by Charles Dickens, Being Shakespeare by Jonathan Bate, The Man Jesus by Matthew Hurt, and Inside Wagner’s Head.

In 2004, Simon appeared on a Comic Relief episode of Little Britain. He also hosted the London Gay Men's Chorus Christmas Show, Make the Yuletide Gay at the Barbican Centre in London. He is currently one of the patrons of the Michael Chekhov Studio London and of the London Oratory School Scholarship. In 2007, he threatened to resign this last post over controversy surrounding the Terrence Higgins Trust, a charity for which Callow is also a patron.

Notable recent acting work has included his performance as Count Fosco, the villain of Wilkie Collins's The Woman in White, both in film and on stage; as Pozzo in Beckett's Waiting for Godot opposite Sir Ian McKellen, Sir Patrick Stewart and Ronald Pickup; and as the psychiatrist in Chichester Festival Theatre's production of Peter Shaffer's Equus. Earlier this year, he appeared in Matthew Hurt’s one-man play, The Man Jesus, at the Lyric Theatre in Belfast.  

Amongst his extensive voice work, highlights have been playing the role of Wolfgang in Shoebox Zoo and narrating the audio book of Robert Fagles' 2006 translation of Virgil's The Aeneid.

Simon has written acclaimed biographies of Charles Laughton and Orson Welles. He is currently at work on the third volume of his life of Welles. He followed up Being an Actor with Shooting the Actor, which tackles the subject of acting for the screen. My Life in Pieces, an alternative journalistic autobiography, incorporates some of his widely read journalism with memoir. He has also written several shorter books: Shakespeare on Love, Oscar Wilde and His Circle, Dickens' Christmas, and The Night of the Hunter. His most recent publication is his highly acclaimed 2012 biography of Dickens, Charles Dickens and the Great Theatre of the World. In 2013, he appeared in his first work for the stage, Inside Wagner’s Head, at the Royal Opera House. 

CBE (For Sevices To Acting) Awarded In 1999

Honorary Doctorate Lld: Birmingham University

Honorary Doctorate Lld: Queens University, Belffast

Honorary Doctorate Lld: The Open University