When the World Hockey Association began operations in 1972, it made no attempt to hide the fact that it wanted to draft and sign 18-year-old players, even though the NHL's limit was 20. Signing teenagers was a problem because any good 18-year-old was playing junior hockey in Canada, and every junior was under contract to that team for the duration of his career -- that is, until he turned 20 and was drafted by an NHL team. However, the WHA argued that the junior contracts were null and void when a player turned 18, the legal age in Canada, and that any contract signed by a minor wasn't financially beneficial to him and therefore inapplicable in a court of law.
Tonelli turned 18 near the end of the 1974-75 season with the Marlies, and his agent, Gus Badali advised him, to stop playing by his birthday. At the same time, he was being pursued by the Houston Aeros of the WHA, and all hell broke loose when Tonelli did in fact leave the Marlies and signed with the Aeros on his birthday. The Ontario Hockey Association sued him for breach of contract, and a bitter battle was fought in the courts over the summer. In the end, Tonelli was ruled an 18-year-old adult and free to do as he chose.
That fall, Tonelli played pro in Houston with linemates Gordie and Marty Howe. The Aeros, however, folded after three more years in Houston, and the New York Islanders signed Tonelli in the summer of 1978. He worked hard on his game every day and quickly established himself as a necessary piece of the Islanders' Stanley Cup puzzle.
Tonelli established himself as a digger, someone who could go into the corners and come out with the puck. But as he gained confidence, he also became more aggressive in open ice and began to score more. Not coincidentally, the Islanders also started to win. Tonelli was part of the four Cup victories the team won from 1980 to 1983, but in some ways his career culminated in the fall of 1984 when he played for Canada in the Canada Cup, an invitation he almost turned down.
He not only made the team, he had nine points, including a key assist on Mike Bossy's goal in overtime of the semifinal. Canada won the championship and Tonelli was named the tournament's best player. He then rejoined the Islanders and had his best season ever, scoring 42 goals and 100 points.
Midway through the 1985-86 season, he was traded to Calgary and two and a half years later he was in Los Angeles, far removed geographically and psychologically from the Cup-winning teams of New York. He finished his career in 1991-92, playing a handful of games with Chicago and Quebec, and then retired after having played 1,028 NHL games.