[citation needed] Mathias began his acting career by appearing on the television screen in a small role on an episode of the cult BBC TV series Survivors, in 1977.

Mathias's play Cowardice was produced at the Ambassadors Theatre in London in August 1983, starring Ian McKellen, Janet Suzman and Nigel Davenport[6] and received poor reviews.

After receiving critical acclaim, Mathias directed a full run in 1990 at the National Theatre with McKellen alongside Paul Rhys and Christopher Eccleston,[8] winning the City Limits Award for Revival of the Year.

In 1994, Mathias won the London Critics Circle Theatre Award for Best Director for Noël Coward's Design for Living (with Rachel Weisz, Clive Owen, Paul Rhys and Rupert Graves) and Jean Cocteau's Les Parents terribles, starring Sheila Gish, Frances de la Tour, Alan Howard and Jude Law.

The latter transferred to the Ethel Barrymore Theatre on Broadway in April 1995 as Indiscretions, with Law joined by Kathleen Turner, Eileen Atkins, Roger Rees and Cynthia Nixon.

Other London directorial credits include Antony and Cleopatra, starring Alan Rickman and Helen Mirren, in 1998, and Tennessee Williams' Suddenly Last Summer with Sheila Gish in April to July 1999 at the Comedy Theatre.

As a youngster in South Wales, Mathias said, he used to listen to the original Broadway recording of the show and sing "The Ladies Who Lunch" with friends: "I couldn't believe the songs, the cynicism, the sexuality."

For the 2004 Christmas season, Mathias directed the pantomime Aladdin at the Old Vic in London, with McKellen as Widow Twankey alongside Maureen Lipman, Roger Allam and Joe McFadden.

[18] He returned to the US to direct Anton Chekhov's The Cherry Orchard, with Annette Bening, Alfred Molina and Lothaire Bluteau, which opened at the Mark Taper Forum in Los Angeles in February 2006.

He began 2008 by directing a revival of Ring Round the Moon, Christopher Fry's adaption of Jean Anouilh's comedy, L'Invitation au Château, starring Angela Thorne at the West End Playhouse Theatre (opening in February 2008).

[22] He directed McKellen and Patrick Stewart in Samuel Beckett's Waiting for Godot, which toured the UK in early 2009 before opening at the Theatre Royal Haymarket, London in May 2009.

[25] Mathias directed Waiting For Godot and No Man's Land in repertory on Broadway at the Cort Theatre again starring Ian McKellen and Patrick Stewart.

The plays ran from November 2013 to 30 March 2014 to rave reviews – with Ben Brantley of The New York Times calling them "Absurdly Enjoyable" and "...these productions find the pure entertainment value in existential emptiness.