Mary Tamm was born in Dewsbury, West Riding of Yorkshire, and is of Estonian descent. She is a graduate and an associate member of the Royal Academy of Dramatic Art.
Tamm began acting on the stage with the Birmingham Repertory Company in 1971. She moved to London in 1972 and appeared in the musical Mother Earth. Before her association with Doctor Who, Tamm acted in several films, most notably The Odessa File (1973) 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, believing that the role was merely that of the "damsel in distress". She changed her mind when assured by the producers that Romana would be a member of the Doctor's own race and therefore as capable as he. Tamm left the programme after only one season because she felt that the character had reverted to the traditional assistant role and could not be developed further. 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 a new actress, but she was not invited to do so. Some sources state that pregnancy was the reason that she was not asked to return, which Tamm rebuts as a false rumour invented by producer John Nathan-Turner.
After leaving the series, Tamm took leading roles in two BBC1 dramas, The Treachery Game (1980) and its sequel The Assassination Run (1981) alongside Malcolm Stoddard. A leading role in the sitcom The Hello, Goodbye Man opposite Ian Lavender was produced in 1984, for BBC2.
Since then, Tamm has played 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 Patience in the second series of the Gallifrey audio plays produced by Big Finish Productions. Her first such appearance was in Gallifrey: Lies (2005). She has also appeared (as herself) in a special feature in the 2007 DVD boxed set release of The Key to Time, discussing her experiences on the programme.
In August 2009, Tamm made a brief, one-week appearance as Orlenda, in EastEnders.
Mary Tamm died on 26 July 2012. She had been suffering from cancer for 18 months.
Tamm married Marcus Ringrose in 1978. They have one daughter and a grandson.
Her autobiography titled First Generation was published in September 2009 by Fantom Films.