Sheahan was Headmaster of Melbourne Grammar School from 1995 to 2009, having been Principal of his old school Geelong College from 1986 to 1995, Second Master of St Peter's College, Adelaide from 1984 to 1985, and Housemaster of Manifold House, Geelong Grammar School from 1979 to 1983.
He held the post of Vice President of the Melbourne Cricket Club from 2003 until he was elected President on 17 February 2011.