The Welsh-born Mathias began his showbiz career as an actor in the late 1970s in fringe productions. He made his film debut in a small role in Richard Attenborough's overblown WWII would-be epic "A Bridge Too Far" (1977) and went on to appear in small roles in "Priest of Love" (1981), the TV-movie "The Scarlet Pimpernel" (CBS, 1982) and "White Mischief" (1988).
By the early 80s, Mathias had also begun to make inroads as playwright. His first drama, "Cowardice", premiered in the West End starring Janet Suzman and Ian McKellen. Several of his other efforts, notably "Infidelities" and "A Prayer for Wings", premiered at the Edinburgh Fringe Festival before moving to commercial runs. He also adapted David Levitt's novel "The Lost Language of Cranes" (1992) for British television. Mathias added director to his list of credits beginning with "Exceptions" (1988). He staged an acclaimed revival of "Bent" in 1989 as a benefit, co-starring McKellen, Richard E Grant, Ian Charleson and Ralph Fiennes. Based on the critical and audience reaction, the play was revived the following year again with McKellen who was joined by Paul Rhys and Christopher Eccleston, among others. Mathias has since directed highly-praised productions of "Uncle Vanya" (1991), "Design for Living" (1994), "Les Parents Terribles" (also 1994 and on Broadway in 1996 under the title "Indiscretions") and "A Little Night Music" (1995).