She was cast in the role of Romana at the start of Season 16 of Classic Doctor Who, playing a new kind of companion; the first to be the Doctor's true intellectual equal. She left the show after one season due to dissatisfaction with the writing of her character, whom she felt had devolved into a generic "screaming companion".
Her final story, "The Armageddon Factor", did not feature any departure scene for her character, as she had not made a firm decision on whether to stay or leave when filming wrapped, and producer Graham Williams, who had been trying hard to persuade her to stay on, opted to proceed as if she was going to do so. As a result, the first story of the next season began with Lalla Ward, who had appeared as a guest character in Tamm's final story, as a newly regenerated Romana.
Tamm reprised her role as Romana for Big Finish Doctor Who from 2005 onwards. Initially her appearances were confined to Companion Chronicles and the "Audioplay/Gallifrey" series, but shortly before her death in 2012, she recorded a set of Fourth Doctor Adventures alongside Tom Baker, who had finally agreed to work with Big Finish after years of refusing. They were released posthumously in 2013.
Her roles in works with TV Tropes pages:
Film
- The Odessa File (1974) — Sigi
- The Likely Lads (1976) — Christina
- Doghouse (2009) — Meg Nut
TV
- Coronation Street:
- 2 episodes (1973) — Pauline Ogden
- 1 episode (2002) — Diana Black
- Whodunnit: episode "Nothing to Declare" (1975) — Valerie Austin
- Return of the Saint: episode "The Debt Collectors" (1978) — Gerri Hanson
- Doctor Who: 26 episodes / 6 stories (1978-79) — Romana
- Not the Nine O'Clock News: episode "Made in Wales" (1982)
- Jane Eyre: 2 episodes (1983) — Blanche Ingram
- Bergerac: episode "Tug of War" (1984) — Leslie West
- Poirot: episode "The Dream" (1989) — Mrs. Farley
- CASUAL+Y: episode "A Grand in the Hand" (1989) — Virginia Wilson
- The Bill episodes:
- "Now We're Motoring" (1991) — Ms. Crosby
- "Lick of Paint" (2001) — Moira Sutherland
- Brookside: 6 episodes (1993) — Penny Crosbie
- The New Adventures of Robin Hood: episode "Witches of the Abbey" (1997) — Alice
- Crime Traveller: episode "A Death in the Family" (1997) — Mary Chandler
- Heartbeat: episode "Bad Apple" (1997) — Marilyn
- Doctors:
- "God's Will" (2000) ... Lyn Baker
- "Mirror, Mirror" (2006) ... Sylvia Crawford
- "Dying to Please" (2007) ... Jemma Forrester
- Jonathan Creek: episode "Satan's Chimney" (2001) — Vivian Brodie
- Holby City: episode "Crossing the Line" (2006) — Fliss Robson
- Wire in the Blood: "Unnatural Vices": Parts 1 & 2 (2008) — Elektra
- Eastenders: 4 episodes (2009) — Orlenda
Audio Play
- Big Finish Doctor Who:
- Gallifrey: 5 releasesshow list (2005-11) — Romana / Pandoranote
- The Companion Chronicles: 3 releasesshow list (2009-11) — Romana
- The Fourth Doctor Adventures: 7 releasesshow list (2013) — Romana