Duncan Siemens was born on September 7, 1993 in Edmonton, Alberta. He played junior for the Saskatoon Blades of the Western Hockey League and was selected in the first round of the 2011 NHL Entry Draft by the Colorado Avalanche.
Siemens' first full professional campaign would come on 2013-14. The big defender would spend the season in the American Hockey League, suiting up in 46 games for the Lake Erie Monsters.
Injuries limited Siemens to just 54 games for the Monsters in 2014-15, but he did get his first taste of NHL action as the season came to a close. He made his NHL debut on April 10, 2015 for the Avalanche in the club's final game of the season, a 3-2 victory over the Chicago Blackhawks.