Harrison made her acting debut in 1976, in the BBC police drama, Softly Softly, which was a spin-off from the hugely popular Z-Cars. In 1980 she made her film debut, securing a minor role in the 1980 adaptation of The Elephant Man. She went on to appear in many successful television programmes including The Gentle Touch (1982); Q.E.D. (1982); The Cleopatras (1983); Minder (1984); Casualty (1987, 1996 & 2008); Dorothy in London's Burning from 1988–1989; Kavanagh QC (1995) and ITV's A Touch of Frost (1997).
One of her most notable and long running roles, was playing the part of Gloria in the popular BBC sitcom Brush Strokes (1986–1991); remaining in the role for five series. However, it is her role in the BBC soap opera EastEnders (1998–1999) that she is most remembered for. Harrison played Louise Raymond, the selfish mother of Tiffany (Martine McCutcheon) and Simon (Andrew Lynford). Harrison's character was involved in many explosive storylines, including an affair with her daughter's husband Grant (Ross Kemp). This role was not the first time Harrison had appeared in the soap. She had previously had a minor role in 1986, playing the mother of a young boy who was rescued from a speeding lorry by Andy O'Brien (Ross Davidson).
Harrison quit EastEnders in 1999 so she could take a Master's degree in screenwriting. In 2006 she began teaching Script and Screen, a five-week course offered through the adult and community non-accredited provision, at Brighton and Hove College.
Harrison's other acting credits include the cult films Tank Malling (1989) and Human Traffic (1999), and the ITV police drama The Bill (2002). In 2005 she also took part in the Channel 4 series Extreme Celebrity Detox.