Siegenthaler, a defenseman born in Zurich, Switzerland, was taken by the Washington Capitals in the second round (No. 57) of the 2015 NHL Draft after his first full season with his hometown in Switzerland's National League. He played two more seasons in Switzerland before coming to North America for good in the 2017-18 season.
The Capitals assigned Siegenthaler to Hershey of the American Hockey League, where he spent all of the 2017-18 season and started 2018-19. However, he got his first NHL call-up when injuries hit Washington's defense and made his debut with the Capitals against the Columbus Blue Jackets on Nov. 9, 2018. His first point came against the Carolina Hurricanes on Dec. 14, 2018.