In 1977–78, Sittler's 117 points ranked him third in regular-season scoring behind Guy Lafleur and Bryan Trottier, and also earned him a Second Team All-Star selection.

Imlach and Ballard both had strained relations with NHLPA executive director Alan Eagleson, who, as a player agent, represented more than a dozen Leafs, including Sittler and his best friend and linemate, Lanny McDonald.

When Sittler and goaltender Mike Palmateer agreed to appear on the TV show Showdown, as negotiated by the NHLPA, Imlach went to court to obtain an injunction to stop them.

But even with Imlach gone, Sittler's relationship with the Leafs worsened to the point where he told Ballard and acting general manager Gerry McNamara at the end of November he would waive his no-trade clause if he was sent to the Flyers or the Minnesota North Stars.

In the first week of December, Eagleson agreed to terms with Flyers' owner Ed Snider and North Stars' general manager Lou Nanne.

On January 5, 1982, on advice from his physician, Sittler walked out on the Leafs, saying he was "mentally depressed" because a trade was taking so long to complete.

Finally, on January 20, 1982, the 31-year-old Sittler was traded to the Flyers for Rich Costello plus the Hartford Whalers' second-round pick in the 1982 draft (used by the Leafs to select Peter Ihnačák), and future considerations, which ended up being Ken Strong.

On the day the announcement was to be made—Sittler even had a brief speech prepared[11]—he was instead told by Flyers' newly appointed general manager Bobby Clarke he had been traded to the Detroit Red Wings for Murray Craven and Joe Paterson.

He had an unproductive season, struggling to get ice time under coach Nick Polano, and finishing the year with the worst goals-per-game average of his NHL career.

In 1991, a year after Ballard died, he rejoined the Maple Leafs organization as a consultant under general manager Cliff Fletcher, also performing public relations duties for the team on an ongoing basis.

[13] In a ceremony, Sittler, with his three grown children at his side, paid tribute to his wife Wendy, who had died of cancer the previous year.

His daughter Meaghan Sittler played four seasons at Colby College and on the United States national women's hockey team.

In July 1980, Sittler presented Terry Fox with his NHL All-Star Game jersey in Nathan Phillips Square in front of a crowd of over 10,000 supporters.