One of the few players ever drafted out of Canadian university hockey, MacAdam was selected 55th overall by the Philadelphia Flyers in the 1972 NHL Amateur Draft after starring for the University of Prince Edward Island. He turned pro after being drafted, and spent most of his first two professional seasons with the Richmond Robins, Philadelphia's AHL affiliate. He made his NHL debut in the 1973–74 campaign, appearing in 5 games for the Flyers without scoring a point. He was also recalled for the NHL playoffs, and made his NHL playoff debut in the clinching game of the 1974 Stanley Cup finals as the Flyers won the championship. By virtue of that appearance, he is considered a member of the championship team although he had only appeared in 6 NHL games to that point, and received a Stanley Cup ring for his efforts. However, AL MacAdam's name cannot be found any where on the Stanley Cup.
Following the season, MacAdam was dealt to the California Golden Seals in a deal which saw star sniper Reggie Leach going the other way. He was able to step as a regular into a weak Seals lineup, and recorded a fine rookie season in 1974–75 with 18 goals and 43 points. In 1975–76, he emerged as the Seals' top player, leading the team with 32 goals (including 4 shorthanded markers) and 63 points, and was selected to play in the 1976 NHL All-Star Game.
For the 1976–77 season, the Seals were moved and became the Cleveland Barons. MacAdam continued to excel, again recording 63 points, and was selected to play in his second consecutive All-Star game. In 1977–78 he was named team captain, but slumped to totals of just 16 goals and 48 points.
MacAdam became a member of the Minnesota North Stars in 1978–79 when the hapless Cleveland franchise merged with Minnesota. He would have a solid first season with the North Stars, finishing 2nd on the team in scoring with 58 points despite missing 11 games due to injury. One of the league's most durable players, he had not missed a game to that point in his career, and only missed 10 games combined in his other 11 full NHL seasons. During that season, he also forged a solid partnership with rookie centre Bobby Smith, and the two would be regular linemates over the next several seasons.
In 1979–80, MacAdam would have his finest NHL campaign, as he posted 42 goals and 51 assists for 93 points, leading the North Stars in all three categories. He finished 12th in overall NHL scoring, and was awarded the Bill Masterton Trophy in 1980 for perseverance and dedication. He was also named Minnesota's MVP and Most Popular Player following the season. Appearing in the playoffs for the first time since 1974, he also led the North Stars in playoff scoring with 16 points in 15 games as they reached the Stanley Cup semi-finals. In the 1980 playoffs, he would score the deciding goal in Game 7 of the quarterfinals, that would eliminate the defending Stanley Cup champion Montreal Canadiens.
MacAdam's production returned to its previous level in 1980–81, as he finished 3rd on the North Stars in scoring with 21 goals and 60 points. However, he would have another sterling playoff performance with 19 points in 19 games as Minnesota reached the Stanley Cup finals before losing to the New York Islanders. In 1981–82, he recorded 18 goals and 43 assists for 61 points in what would be his last quality offensive season.
Always a strong defensive player, MacAdam's offensive opportunities would be severely reduced after 1982 with the arrival of star rookie Brian Bellows and he would spend the remainder of his career as a 3rd-line checker and defensive forward. His production fell to a career-low 11 goals and 33 points in 1982–83, but he rebounded to record 22 goals for the North Stars in 1983–84.
For the 1984–85 campaign, MacAdam was dealt to the Vancouver Canucks in exchange for Harold Snepsts. He had a solid season with 14 goals and 34 points, but could not meet the expectations of being traded for Snepsts, who was one of the most popular players in Canuck history. He retired shortly after being assigned to the minors at the start of the 1985–86 season.
MacAdam finished his career with totals of 240 goals and 351 assists for 591 points in 864 games, along with 509 penalty minutes. His 591 points was the most ever for a player from Prince Edward Island, just ahead of Bob MacMillan, although that record was broken by Brad Richards of the Dallas Stars in 2010.
During his playing days, MacAdam was also respected as a tough and capable fighter "who many left alone" in the words of Clark Gillies of the New York Islanders.
At the conclusion of his career, MacAdam was appointed head coach and assistant athletic director at St. Thomas University in Fredericton, New Brunswick. He spent 11 years at St. Thomas, and was named AUAA Coach of the Year in 1995–96.
He returned to pro hockey in 1997 to accept a job as head coach of the St. John's Maple Leafs of the American Hockey League in 1997. After three successful seasons there he returned to the NHL in 2000–01, joining the coaching staff of the Chicago Black Hawks, and spent four seasons as an assistant coach in Chicago.
After leaving Chicago he was hired as head coach of the Halifax Mooseheads in the Quebec Major Junior Hockey League, and spent two seasons there before retiring after the 2005–06 season. He currently resides in Prince Edward Island, and is employed by the Buffalo Sabres as a scout.Won Stanley Cup with Philadelphia Flyers (1974)
Played in NHL All-Star Game (1976, 1977)
Won Bill Masterton Trophy (1980)
Named California Golden Seals MVP (1975–76)
Led California Golden Seals in scoring (1975–76)
Named Minnesota North Stars MVP (1979–80)
Led Minnesota North Stars in scoring (1979–80)
Named AUAA Coach of the Year (1995–96)