Wescott was born in Durham, North Carolina, and lives in Carrabassett Valley, Maine. Growing up Seth Wescott went to Mount Blue regional school district in Farmington Maine. His father Jim Wescott was the Track and Cross Country coach at Colby College. He began snowboarding at age 10, but had also grown up skiing. In 1989, after competing in both sports for a few years, he stopped skiing to focus mainly on snowboarding. Wescott attended Carrabassett Valley Academy where he studied and trained with fellow Olympians Bode Miller, Jeff Greenwood, Kirsten Clark and Emily Cook. He started out in Sugarloaf USA in Carrabassett Valley, Maine.
In his Olympic debut, at the 2006 Winter Olympics in Torino, Italy, Wescott won gold in the snowboard cross  as the first Olympic champion in the event. Having won gold, Wescott was invited to meet then president George W. Bush but turned down the offer, citing his opposition to Bush's foreign and domestic policies. At the 2010 Winter Olympics in Vancouver, Canada, Wescott successfully defended his Olympic gold. Wescott, who started off the race in fourth, slowly advanced throughout the field until the end, when he narrowly defeated hometown favorite Mike Robertson on the final jump.
Wescott co-owns The Rack, a restaurant and bar near Sugarloaf that caters food and drinks to skiers and snowboarders.
On February 25, 2010, Wescott appeared on The Colbert Report. In 2012, he participated in Fox's dating game show The Choice.