Ersson's hockey career took off after he signed a two-year, $2.9 million contract with the Philadelphia Flyers on Aug. 4, 2023. The fifth-round pick (No. 143) in the 2018 NHL Draft emerged as Philadelphia's starting goalie and in 2024-25 was named to Sweden's roster for the 4 Nations Face-Off as an injury replacement for Jacob Markstrom of the New Jersey Devils.

Ersson, a bronze medalist at the 2024 IIHF World Championship, played for the Flyers and Lehigh Valley of the American Hockey League in 2022-23. He went 24-17-1 with a 2.84 goals-against average, .900 save percentage and one shutout in 42 AHL games and 6-3-0 with a 3.07 GAA, .899 save percentage and one shutout in 12 NHL games.

Ersson also had international success when he started for Sweden at the 2018 IIHF World Junior Championship, going 3-1-0 with a 2.23 GAA and .922 save percentage. He made 28 saves for his first NHL shutout, a 4-0 win at the Buffalo Sabres on Jan. 9, 2023, to become the 14th Flyers rookie to not allow a goal and first since Anthony Stolarz on Jan. 19, 2019, and needed the fifth fewest games (five) with Philadelphia behind Robbie Moore (one game, 1978-79), Stolarz (two in 2016-17), Bob Froese (three in 1982-83) and Doug Favell (three in 1967-68).

Ersson became the 11th goalie to win each of his first six decisions with 32 saves in a 4-3 victory at the Calgary Flames on Feb. 20, 2023. He joined Ray Emery (9-0-0), Martin Jones (8-0-0), Viktor Fasth (8-0-0), Froese (8-0-0), Dan Ellis (7-0-0), Wayne Thomas (7-0-0), Spencer Knight, (6-0-0) Frederik Andersen (6-0-0), Damian Rhodes (6-0-0), Ken Dryden (6-0-0) and Frank McCool (6-0-0).

Ersson signed his entry-level contract June 3, 2021, after two seasons with Brynas in the Swedish Hockey League. He went 27-9-0 with a 1.95 GAA and .933 save percentage for Vasteras in Allsvenskan in 2018-19.