He was a Labor Party member of the New South Wales Legislative Assembly from 1962 to 1980, representing the electorate of Bankstown.
Kearns entered state politics at the 1962 election, winning the safe Labor seat of Bankstown upon the retirement of Spence Powell.
He was a strong advocate on issues of social welfare, and was a popular member in what was at the time a rapidly growing working-class area with a high proportion of migrants.
He served for a period as Shadow Minister for Youth and Community Services prior to Labor's victory at the 1976 election, but narrowly missed out on a ministerial position afterwards.
He had announced his intention to retire at the 1981 election when he died suddenly while on a holiday in the Northern Territory in July 1980.