After spending the first half of his first professional season with the Oklahoma City Blazers, MacLeish was involved in a three-way deal which sent him; Bruce Gamble, Dan Schock, and a 1st round pick to the Philadelphia Flyers, Bernie Parent and a 2nd round pick to the Toronto Maple Leafs, and Mike Walton to Boston.

In the 1971–72 season he saw his output drop considerably to a single goal, and consequently split the year between the Flyers and their AHL affiliate the Richmond Robins.

[6] He scored the only goal in the series' sixth and final game, and narrowly missed out on winning the Conn Smythe Trophy to his teammate, Bernie Parent.

[7] After another successful regular season in 1974–75, notching 38 goals and 41 assists MacLeish went on to lead his team again in playoff scoring as they won a second consecutive championship.

He managed to accumulate 22 goals and 23 assists in the regular season, but, was unable to play in the playoffs, which saw the Flyers swept in four games by the Montreal Canadiens.

[6] In a game against the Los Angeles Kings in April 1978, MacLeish narrowly avoided serious injury suffering a cut neck requiring 80 stitches when he slid into the skate of center Marcel Dionne.

After the 1980–81 season, the Flyers traded MacLeish, Blake Wesley, and Don Gillen to the Hartford Whalers for Fred Arthur and Ray Allison.

In 2016, sportswriter Bill Fleischman recalled, "Forty-two years later, I can still see him gliding across the opponent's blue line with the puck, often swerving left, leaning as if he might tumble to the ice and flicking a wrist shot past the goalie.

[15] On May 11, 2016, MacLeish's daughter announced her father had been "battling multiple medical problems" for the last 6 weeks in a Philadelphia hospital.