She was the sister of naturalist and broadcaster Tom Weir. Born in Glasgow and brought up in the Springburn area of the city, Weir began in amateur dramatics. In her early professional career, she was a well-known radio actress, featuring in many comedy shows, such as ITMA. She made her film debut in 1949, and had a regular role as the housekeeper, Aggie McDonald, in the radio and television sitcom Life With The Lyons.
She also appeared in a series of television advertisements for Flash the household cleaning agent.
In 1969 she appeared in The Prime of Mission Jean Brodie starring Dame Maggie Smith. She and Helena Gloag played the Kerr sisters, the sewing mistresses of Marcia Blaine School for Girls.
In 1970 Weir and Gloag reprised their collaboration in Scrooge, playing old sisters in debt to Mr Scrooge, played by Albert Finney. In the 1970s she was one of the presenters of Teatime Tales, a television series broadcast by STV in which she recalled her childhood.
The series also featured Lavinia Derwent and Cliff Hanley.
In the 1980s, she lampooned this homely image in the comedy series Victoria Wood As Seen On television and appeared in a popular video for the Bluebells 1983 hit Young At Heart. After her death, Molly Weir"s ashes were scattered on the banks of Loch Lomond, a favourite holiday location. And almost all her estate (of nearly £19 million), was bequeathed to charities.