Sieloff was drafted in the second round, 42nd overall, by the Calgary Flames in the 2012 NHL Entry Draft. On July 19, 2013, the Flames signed Sieloff to a three-year two-way contract. On April 9, 2016, Sieloff scored his first NHL goal in his first NHL game, which also turned out to be the game-winning goal, on Darcy Kuemper of the Minnesota Wild in a 2–1 victory. On June 27, 2016, Sieloff was traded by the Flames to the Ottawa Senators in exchange for Alex Chiasson. On September 25, 2016, Sieloff hit fellow Senator Clarke MacArthur hard during a training camp scrimmage at the team's annual Fan Fest in an incident that left MacArthur with a concussion. Bobby Ryan immediately dropped his gloves and went after Sieloff, and on his next shift Chris Neil also attempted to get retribution. Sieloff was subsequently removed from the game as a precaution. He was subsequently sent down to the AHL and did not receive a call up that season. MacArthur, who had a history of concussions prior to the incident, would only play 23 more NHL games in his career following the hit. Despite the incident, he signed a one-year, two-way contract with the Senators in the summer before the 2017–18 season began. He received his first call up on March 20, 2018, to play in a game against the Florida Panthers. In his debut, he scored a goal in the 7–2 loss, becoming the first player in NHL history to score a goal in their debut for two different teams. He was reassigned to the AHL on March 21, 2018. Following being named Belleville's defenceman of the year, Sieloff signed a two-year, two-way contract with the Senators. Prior to the 2018–19 season, Sieloff was named an alternate captain for the Belleville Senators along with Paul Carey and Ben Sexton. He ranked third in appearances on the blueline for Belleville, posting 9 points in 45 games, before he was traded by the Senators to the Anaheim Ducks in exchange for Brian Gibbons on February 25, 2019. Assigned to report directly to the San Diego Gulls of the AHL, Sieloff completed the regular season with 1 assist in 14 appearances. He made his AHL playoff debut with the Gulls, going scoreless in 3 games. In his final year under contract with the Ducks, Sieloff continued his tenure with the Gulls in the 2019–20 season. He registered just 1 goal in 19 games with San Diego before he was traded by the Ducks to the Tampa Bay Lightning in exchange for Chris Mueller on December 31, 2019.
