After his retirement from playing, Sumich coached South Fremantle between 2000 and 2001, before signing as an assistant coach at West Coast, where he served in that role from 2002 to 2011, before joining Fremantle as an assistant coach. In total, he kicked 514 goals, a club record, from 150 games for West Coast, leading the club's goalkicking on a record seven occasions. After kicking 111 goals during the 1991 season, Sumich became the first Eagles player, and the first left-footer, to kick more than 100 goals in a season.