During the campaign, Alt's NHL rights were traded by the Hurricanes to the Philadelphia Flyers on January 13, 2013, along with Brian Boucher, for Luke Pither.

[9] He immediately joined the Flyers' American Hockey League affiliate, the Adirondack Phantoms, on an amateur try-out contract to finish the 2012–13 regular season.

Continuing his upward development the Flyers, Alt was recalled on emergency, and made his NHL debut, playing 9:25 with Philadelphia in a 3-2 shootout loss to the San Jose Sharks on March 28, 2015.

[11] In the final year of his entry-level contract, Alt remained exclusively in the AHL with the Phantoms in the 2015–16 season, appearing in a further 72 games paired alongside Samuel Morin in a shutdown role, collecting 4 goals and 19 points.

As an impending restricted free agent, Alt was re-signed to a one-year, two-way contract to return for his sixth season within the Flyers organization on June 27, 2017.

[11] As the Phantoms' best defenseman to start the year, Alt returned to the NHL for the first time in three seasons, as he was again used on an emergency basis in a 4-3 overtime defeat to the Arizona Coyotes on October 31, 2017.

[16] As the Flyers' reserve depth defenseman, Alt was a frequent healthy scratch and failed to feature in a game in over a month before he was placed on waivers in order to return to the AHL.

[17] With the Avalanche vying for a playoff position, he assumed the same role with his new club and remained in the press box until making his belated debut with Colorado in a 2-1 shootout victory over the Vegas Golden Knights on March 24, 2018.

[18] He played in 7 more games with the Avalanche to end the regular season and remained a healthy scratch in their first-round defeat to the Nashville Predators in the playoffs.

[21] In the pandemic delayed 2020–21 season, Alt made a return to the NHL, featuring in 2 games with the Kings before he was assigned to their AHL affiliate, the Ontario Reign, for the remainder of the campaign.

[24] Alt's father, John, played left tackle in for the Kansas City Chiefs of the National Football League (NFL).