Born in Chelsea, the daughter of Sir George Catlin and Vera Brittain, the pacifist and feminist writer, Williams joined the Labour Party and was elected to Parliament in 1964, later becoming a Cabinet minister. Increasingly out of sympathy with the leftward drift of Labour, particularly after 1979 when she lost her seat in the House of Commons, in 1981 she joined with Roy Jenkins, David Owen and Bill Rodgers, the so-called Gang of Four, to form the new Social Democratic Party and was elected as MP for Crosby at a dramatic by-election later that year. Williams supported the Alliance with the Liberal Party and later the merger which formed the Liberal Democrats. She became a Lib Dem life peer as Baroness Williams of Crosby in 1993 serving as leader in the Lords from 2001-2004. In 2016 she retired from the Lords and in 2017 was appointed a Companion of Honour for services to political and public life. She died in April 2021, aged 90.