The 1972–73 season, during which the Flyers earned the nickname "the Broad Street Bullies" proved to be a breakout year for MacLeish as he became the first member of the Flyers to ever score 50 goals in a single campaign. He added 50 assists that year, to bring his points total to 100. This was enough to finish fourth in league scoring, only a single point behind Bobby Orr. In the Stanley Cup playoffs, the Flyers won their first playoff series against the Minnesota North Stars and faced the heavily favored Montreal Canadiens in the semi-final round. MacLeish and the Flyers stunned the Canadiens winning the opening game in Montreal when the Flyers' center intercepted an errant Frank Mahovlich pass (Mahovlich lost the puck in a legendary "puddle of water" on the ice) and scored in overtime. The Flyers pushed the Canadiens into overtime in game 2 as well, but, would lose that game 4–3 and would go on to lose the series 4 games to one.

In the 1973–74 season, MacLeish's regular season scoring dropped slightly, as he scored 32 goals and added 45 assists. In the playoffs, however, he led all scorers with 13 goals and 9 assists as the Flyers claimed their first ever Stanley Cup. 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.

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. This championship marked the last time the trophy was raised by a team consisting solely of Canadian born players. MacLeish's 1975–76 season was marred by injury, as he only played in 51 games. 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.

The following year saw MacLeish lead the Flyers in scoring for the first time in his career, tallying 49 goals and 48 assists. It also was the first year in which he earned an invitation to the NHL All-Star Game, a feat which he duplicated the following year. 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. Several days later, he was back in the lineup. He joked later that he smoked a cigarette in the locker room afterward and smoke came out his throat.

After the 1980–81 season, the Flyers traded MacLeish, Blake Wesley, and Don Gillen to the Hartford Whalers for Fred Arthur and Ray Allison. During the 1981–82 season, the Whalers traded MacLeish to the Pittsburgh Penguins for Russ Anderson. Pittsburgh released MacLeish during the 1982–83 season, and he played briefly in Switzerland. He rejoined the Flyers as a free agent for the 1983–84 season, and was traded during the season to the Detroit Red Wings for future considerations. He retired at the end of that season.