Born Prunella Margaret Rumney Illingworth in Sutton Abinger, Surrey, she he has had a long and distinguished career as an actress mostly in comic roles. Her early film roles included Pride and Prejudice and Hobson's Choice.

Her first career break came with the early 1960s sitcom, Marriage Lines starring opposite Richard Briers. She has had major roles in BBC Radio 4 sitcoms, most notably After Henry, Smelling of Roses and Ladies of Letters; on televison she starred in the LWT/Channel 4 series Mapp & Lucia based on the bestselling novels by E. F. Benson. More recently she was seen in a series of Tesco supermarket commercials as a domineering mother, Dottie Turnbull, with Jane Horrocks as her long-suffering daughter.

Scales narrowly missed out on the role of "Eth" in The Glums, part of Take It From Here, written by Frank Muir and Denis Norden, but was narrowly beaten by June Whitfield.

She is married to the British actor Timothy West, and has two sons, their eldest is the actor and director Samuel West. In 2003, she appeared as Hilda, alias "she who must be obeyed", wife of Horace Rumpole in a series of four BBC Radio 4 plays, with her husband playing her fictional husband. They are both ambassadors for SOS Children's Charity .

Prunella Scales and Timothy West toured Australia at the same time in different productions. Prunella Scales appeared in a one-woman show called "An Evening with Queen Victoria".

Scales is a supporter of the Labour Party, and appeared on a Labour party political broadcast during the 2005 UK general election campaign.

Her authorized biography, "Prunella" written by Teresa Ransom, was published by John Murray in 2005 .