Tamm began her acting career on the stage with the Birmingham Repertory Company in 1971, moving to London in 1972 and appearing in the musical Mother Earth. Before her association with Doctor Who, Tamm acted in several motion pictures, most notably The Odessa File (1974) and The Likely Lads (1976). In 1981 she took the part of Rhoda Dawes in Agatha Christie's Cards on the Table at London's Vaudeville Theatre.
Tamm was not initially interested in playing a companion to the Doctor as she believed that the role was merely that of the "damsel in distress". However, she changed her mind when assured by the producers that Romana would be different. Romana was a Time Lady, a member of the Doctor's own people and therefore as capable as the Doctor. She left the programme after only one season as she felt that the character had reverted to the traditional assistant role and could not be developed further. The role of Romana was assumed by Lalla Ward. In a 2007 interview, Tamm stated that she was willing to film a regeneration sequence to allow a smooth transition between her tenure and that of Ward, but was not invited to do so.
Since then, Tamm has acted in both film and television, playing the characters of Penny Crosbie in the soap opera Brookside from 1993 to 1996, Yvonne Edwards in the BBC drama Paradise Heights (2002), as well as guest roles in many other television programmes. Tamm returned to the role of Romana in the second series of the Gallifrey audio plays produced by Big Finish Productions. Her first appearance in these was in Gallifrey: Lies (2005). She has also appeared (as herself) in the Special Features of the 2007 DVD boxed set release of The Key to Time, discussing her season long stint on the programme.
In August 2009, Tamm made a brief, one-week appearance as Orlenda, in EastEnders.
Rare Signed 10 x 8 Photograph of Mary Tamm. This is an orginal autograph and not a copy.