An aboriginal Canadian who grew up in Riverton, Manitoba, Leach was raised by his paternal grandparents. One of the greatest scorers in the history of the Western Canada Junior Hockey League, Leach found the back of the net 188 times in three seasons with the Flin Flon Bombers. He led the WCJHL/WHL in goal-scoring twice, including a remarkable total of 87 goals in 1967-68. Leach was a junior teammate of Bobby Clarke, a factor that later changed the course of his NHL career. Following each of his junior seasons, he was placed on the WCJHL/WHL First All-Star Team.
After graduating from junior, Leach was the third player claimed in the 1970 Amateur Draft when the Boston Bruins called his name. The Beantowners were in the midst of winning the Stanley Cup twice in three years and were too deep in talent to give the youngster a fair shot at the pros.
On February 23, 1972, Leach was part of the package sent by Boston to California to acquire Carol Vadnais. The Golden Seals were blatantly overmatched most nights, but Leach's talent began to shine through the depression of losing. In 1972-73 and 1973-74, he recorded consecutive 20 goal seasons. In the second of these years, he formed the team's top line with Walt McKechnie and Joey Johnston.
A few days after winning their first-ever Stanley Cup, the Philadelphia Flyers took a giant step toward repeating the triumph when they fleeced the Seals in a deal for Leach. In 1974-75, Leach teamed with Clarke and Bill Barber to score 45 goals and earn the respect of the tough fans in the City of Brotherly Love. While helping the Flyers repeat as Cup winners, Leach scored eight goals in 17 post-season games.
It was in the Flyers' failed attempt at a third triumph in the playoffs that Leach's star shone the brightest. During the 1975-76 season, he scored 61 regular-season goals but saved his best for the post-season. Leach's 19-goal effort in 16 games earned him the Conn Smythe Trophy even though the Montreal Canadiens in the Stanley Cup finals swept his team. Of further significance was the fact that the Riverton Rifle scored 80 combined goals (regular-season and playoffs) to break Phil Esposito's standard set in 1970-71.
Incredibly, in 1979-80 the wily veteran scored 50 times and helped the Flyers set an NHL record by going undefeated in 35 consecutive games from October 14 to January 6. More important, Leach took on defensive responsibilities and killed penalties for the first time in his career. He also scored 16 points while helping Philly reach the Stanley Cup finals, where they lost to the New York Islanders in six games.
The fleet winger registered 60 goals during his last two seasons in Philadelphia before joining the Detroit Red Wings as a free agent prior to the 1982-83 season. Leach scored his last 15 NHL goals wearing the famous winged wheel. In 1983-84, he skated for the Montana Magic of the Central Hockey League before bringing his pro career to a close. Leach finished with 381 goals in 934 regular-season games while earning a reputation as one of the top snipers of his day.