Born Sally James in Horsham, West Sussex, the daughter of a doctor, Knyvette took acting lessons while running a French restaurant in the Kings Road, London, then was in rep for seven years, acting mostly on stage, including playing Shakespeare roles.
On screen Knyvette is best known for her roles as Jenna Stannis in the first two seasons of the British science fiction series Blake's 7 and as Kate Sugden in the soap opera Emmerdale. She later complained of her Blake's 7 role that Jenna had "started off as this really exciting, intergalactic space pirate, but then she became a sort of housewife on the Liberator".
Her other television appearances include The Sweeney, Z-Cars, Who Pays the Ferryman?, Squadron, The Bill, Dalziel and Pascoe, A Touch of Frost, Casualty, Holby City, EastEnders and Coronation Street.
She also voiced the part of Doctorman Allen in the Doctor Who audio adventure Spare Parts and that of Carmilla in the video game Castlevania: Lords of Shadow and its sequel, developed by Mercury Steam.
After leaving Blake's 7 Knyvette studied for an English and drama degree at the University of London. She lives in Fulham, West London.