Shanahan was born in Western Australia, but came to the VFL from Horsham. A defender, he played in a back pocket for Collingwood in the 1925 VFL Grand Final and was on a half back flank in the 1926 VFL Grand Final. He finished on the losing team in both matches.
He went to Carlton for the first part of the 1927 VFL season, playing in the seniors in rounds four and five.
Shanahan returned to Collingwood midway through 1927 and played in the VFL Seconds finals series. Shanahan also played for Collingwood against Western Australia at Subiaco in August, 1927, kicking two goals and best on ground honours!
In 1928 Shanahan played 17 games with Fitzroy.
Shanahan was appointed as captain / coach of the Camberwell Football Club in 1929 but resigned after five games, playing his last game against Yarraville on Saturday, 18 May 1929 and was cleared back to Fitzroy where he played out the 1929 season, playing his first match on Saturday, 22 June 1929 against Richmond.
Shanahan, who worked as a police officer, was appointed coach of Williamstown the following year, in 1931 but was forced to resign before the beginning of the season by the Victorian Chief Commissioner, who ruled that members of the police force could not be football coaches.
Shanahan coached the Henty Football Club in the Albury & District Football League in 1939, only to lose the grand final from a goal kicked after the final siren by a Brocklesby player.
