As a child, Senn was encouraged to play hockey after watching his brother train with Visp. He alternated between forward, defence, and goalie, before eventually settling for goaltender. While growing up in Switzerland, he had limited exposure to the National Hockey League but as he grew older, he began to idolize NHL goaltenders David Aebischer and Martin Gerber. At the age of 16, Senn left the Visp U17 for Davos U17 to combine school and sport. He finished Gymnasium with a high school diploma while continuing to play hockey. Senn helped lead HC Davos to the semi-finals of the 2016 Spengler Cup with a 3–1 win over the Avtomobilist Ekaterinburg. After the 2016–17 season, Davos lost their starting goaltender Leonardo Genoni to SC Bern, which promoted Senn to their starter the following season. He worked alongside his backup Joren van Pottelberghe to help lead HC Davos to the semi-finals, before eventually losing 5–3. Senn played a career high 34 games during the regular season and posted a.910 save percentage. Despite the loss in the semi-finals, Senn's performance impressed the New Jersey Devils who drafted him 129th overall in the 2017 NHL Entry Draft. In August 2017, he was the recipient of the NLA's Youngster of the Year Award. On 17 April 2019, Senn signed a two-year, entry-level contract with the New Jersey Devils. Later that year, he attended the Devils' development camp at the Prudential Center. After attending the Devils' training camp, he was reassigned to their American Hockey League affiliate the Binghamton Devils. Senn eventually made his AHL debut with the Binghamton Devils on 12 October in a 5–4 overtime loss to the Belleville Senators. After posting a.89 save percentage, he was reassigned to their ECHL affiliate the Adirondack Thunder on 23 November, but was recalled before he could play a game. On 20 December, Senn made his NHL debut when he replaced Mackenzie Blackwood in the third period. He made two saves and allowed one goal as the Devils lost 5–2 against the Washington Capitals. In the next game, on 21 December, Senn made his first NHL start, making 35 saves and allowing three goals in a 5–1 loss to the Columbus Blue Jackets.
International play
During the 2017–18 season, Senn was selected as the third goaltender for Team Switzerland during the 2018 IIHF World Championship, behind Genoni and Reto Berra. He played in four exhibition games and recorded a 2.55 goals against average and.914 save percentage. As a member of Team Switzerland, he earned a silver medal after a loss in the gold medal game to Sweden.
