Born in Tumut, New South Wales, the son of the publicans of the hotel at Jugiong, Sheahan attended schools in Tumut and St Patrick's College, Goulburn.
[1] Sheahan gained work as a clerk in the Crown Law Department in 1914 before enlisting in the Australian Imperial Force in 1916, serving in France and Flanders.
Called to the bar in 1930, Sheahan was made a Queen's Counsel in 1953.
Elected as the Labor Party member for the New South Wales Electoral district of Yass in 1941, Sheahan served in parliament until 1973 (from 1950 as the member for Burrinjuck), holding the portfolios of Attorney-General of New South Wales, Minister for Transport and Minister for Health between 1950 and 1965.
[1][2] In 1977, the 1,143 metres (3,750 ft) long "Sheahan Bridge", replaced the Prince Alfred Bridge near Gundagai as the Hume Highway crossing of the Murrumbidgee River.