Mason played Sarah Stevens, the mother in John Hopkins' four-play cycle Talking to a Stranger (1966). A family drama with four characters, the viewpoint of Sarah Stevens was depicted in the fourth play, The Innocent Must Suffer. Her film roles included Charlie Bubbles (1968), Clegg (1970), The Raging Moon (1971), Made (1972), Hennessy (1975), the bullying teacher's wife in Pink Floyd – The Wall (1982), Terry on the Fence (1986), a game show contestant in Victoria Wood Presents (1989), 101 Dalmatians (1996), Love Actually (2003), and the lady who works the sweets trolley in Harry Potter and the Goblet of Fire (2005). She played "The Ancient Booer" in the 1987 film The Princess Bride. Her television roles include appearances on Midsomer Murders, Peak Practice and Juliet Bravo (1982) (Series 1, Ep. 8). She played Mrs Porter in the Granada TV series A Family at War during 1970–71