Twice in the 1960s Bellingham appeared in the Pendley Open Air Shakespeare Festival. She got her big break as a nurse in an ITV afternoon soap opera of the 1970s, General Hospital. She went topless for her roles in Confessions of a Driving Instructor and Sweeney! (1977).
In 1975 she appeared as Nancy, an accomplice to two Australian crooks in The Sweeney episode, The Trojan Bus.
She is best known as the head of the family in the Oxo television adverts during the 1980s. Other prominent roles included the James Herriot drama All Creatures Great and Small (where she was the second actress to play Helen Herriot on television, replacing Carol Drinkwater) and the situation comedy Second Thoughts and its sequel, Faith in the Future.
She starred in the 14-part Doctor Who serial The Trial of a Time Lord (1986) as the Inquisitor. Bellingham reprised the Inquisitor character for the Big Finish Productions audio series, Gallifrey. In 1998 she appeared in Gleb Panfilov's The Romanovs: A Crowned Family as Empress Alexandra.
From 2000 to 2003, Bellingham played Pauline Farnell, the compassionate accountant in At Home with the Braithwaites alongside Amanda Redman and former All Creatures Great and Small colleague Peter Davison. In 2007, she appeared alongside Redman again, this time playing DCI Karen Hardwick in New Tricks. For several months in 2004, she had a recurring role in The Bill as villainess Irene Radford.
She also had a memorable role in the ITV comedy Bonkers playing Mrs. Wadlow, a man-eating suburban housewife who seduces her neighbour's teenage son and turns him into her gigolo. Later that year she filmed guest appearances in episodes of Love Soup and Robin Hood. In October 2007, she appeared in a play entitled Vincent River at the Trafalgar Studios in London. Her performance received critical acclaim, and it was announced on Loose Women in early 2008 that the play would be moving to Broadway in July of that year, although this never actually transpired.
From September 2008 to July 2009, she played the role of Chris Harper in the stage version of Calendar Girls on tour and in the West End. She returned to the show for further tours in 2010, 2011 and 2012.
From 3 September 2012, she will present her own 4pm ITV Food series called My Tasty Travels with Lynda Bellingham in which she travels around the UK in her VW Campervan cooking for people she meets