Her father was a railway clerk who later became a Labour mayor of the County Borough of East Ham. She was a student at East Ham Grammar School.
In the mid-'60s she suffered a broken pelvis and several ribs in a car accident which kept her out of British films. After recovering she was frequently cast in matriarchal roles predominantly on TV.
Kathleen had a son and a daughter with Alaric and a stepdaughter from Alaric's previous marriage.
She was romantically involved with writer/producer Michael Powell, who was responsible for her early stardom in the late 1940s. He was named as co-respondent when she was divorced from her husband, former pilot USAAF pilot John Daniel Bowen, in 1950. She later became friendly with Powell's second wife, the Oscar-winning film editor Thelma Schoonmaker.
Was cast as Lauren Bacall's sister in Lars von Trier's film Dogville (2003), but had to pass on the role due to ill health.