He played with Hobart in the Tasmanian Football League from 1986 to 1990 and was in Hobart's 1990 TFL Premiership side, then also spent some time with Fitzroy pre-season in 1986 and had played with the Fitzroy reserves (at least one game at North Hobart) and became a police officer in Tasmania, when he was granted extended leave to play AFL.
Shanahan played in 22 of 22 matches in the 1997 AFL Premiership Season home and away rounds in which St Kilda Football Club qualified in first position for the 1997 AFL Finals Series, winning the club's 2nd Minor Premiership and 1st McClelland Trophy.
[2] His peak came in 1997 with the Saints when he performed brilliantly at full-back, but during the 1997 AFL Grand Final he suffered from a goal onslaught by Adelaide matchwinners Darren Jarman and Shane Ellen.
Many St Kilda supporters still cringe at then-coach Stan Alves' refusal to move Shanahan off Jarman, who kicked five last quarter goals to win the Grand Final for Adelaide.
After the 1997 season had finished, Shanahan moved to the Melbourne Football Club due to the Saints not granting the then 30-year-old's request for a long-term contract.