She married Percy Clarey, a trade union leader and later a prominent federal MP, on 31 March 1917 at Box Hill.
[1] Husband and wife were both active in the Australian Labor Party, with Kath secretary of the Caulfield branch and president of the Women's Organising Committee in addition to holding membership of the state executive.
She was expelled from the Labor Party in December 1935 together with Maurice Blackburn, having spoken at a rally organised by the Victorian Council Against War and Fascism.
Williams won a position on the state committee of the Communist Party in 1948, which she used primarily to promote equal pay for women; she also advanced this cause as union delegate to the Melbourne Trades Hall Council.
She attended the World Conference of Working Women in Budapest in 1956 as an observer, and wrote a booklet, Equality Will Be Won, about the struggle.