Showing early promise, Siemens was selected to Canada's under-18 team for the 2010 Ivan Hlinka Memorial Tournament, in which they captured the gold medal.

Combining in a formidable partnership, Siemens whilst defensively minded contributed with a career high 43 points in 72 games, earning a selection to the WHL's Second All-Star Team.

Rated as the fourth best North American defenseman by the NHL Central Scouting and despite being one of the youngest eligible in age, Siemens was selected 11th overall in the 2011 NHL Entry Draft by the Colorado Avalanche,[2] with the pick acquired from the St Louis Blues along with Erik Johnson and Jay McClement in exchange for a second round pick, Chris Stewart and Kevin Shattenkirk.

Following a first-round exit with the Blades, Siemens signed an amateur try-out contract with Colorado's AHL affiliate, the Lake Erie Monsters on March 31, 2012.

[1] Having missed 28 games due to injury, on February 5, 2014, Siemens belatedly notched his first professional goal and point against the Grand Rapids Griffins.

[10] Siemens recorded his first NHL point on an assist on Nathan MacKinnon's single goal in a loss to the Calgary Flames on February 24, 2018.

[13] Siemens as a free agent went un-signed over the following summer before accepting an invitation to attend the Calgary Flames 2018 training camp in September.