The 30-year-old held a 10-6-0 record for the Leafs this season along with an .884 save percentage and a 3.42 goals-against average.
Mrazek signed a three-year, $11.4 million contract with a $3.2 million average annual value in the off-season. The netminder has a 10-team no trade list clause included in his contract, according to CapFriendly.
He last played in the 2022 Heritage Classic where he allowed four goals from 37 shots in a 5-2 loss to the Buffalo Sabres.
The Maple Leafs have also signed goaltender Harri Sateri to a one-way, $750,000 contract. The 32-year-old, who played three games for Olympic gold medalists Finland at the 2022 Beijing Games, will need to clear waivers before joining.
The former fourth round pick by the San Jose Sharks in 2008 played nine NHL games with the Florida Panthers in the 2017-18 season.