Dave Keon was named to the Whalers Hall of Fame in 1990.
He joined the Toronto Maple Leafs of the National Hockey League in 1961, winning the Calder Memorial Trophy as the league's top rookie with 20 goals and 45 points in his first season. It was his first of six consecutive 20-goal seasons. In his second year in the NHL, Keon was named to the Second All-Star Team and won the Lady Byng Memorial Trophy as most gentlemanly player, taking only one minor penalty through the entire season.
He repeated as Lady Byng winner in 1962–63, again taking only a single minor penalty all year. He was the Leafs' leading scorer in the 1963–64, 1966–67 and 1969–70 seasons, and the team's top goal scorer in 1970–71 and 1972–73. Keon was considered one of the fastest skaters in the NHL, and one of the best defensive forwards of his era. He would usually play against the opposing team's top centre, and developed a reputation for neutralizing some of the league's top scorers. In 1970–71, he scored eight shorthanded goals, setting an NHL record, which was later broken by Mario Lemieux in 1988–89, with 13 shorthanded goals. Keon won four Stanley Cups with the Leafs, playing on the Cup-winning teams of 1961–62, 1962–63, 1963–64, and 1966–67. In the 1967 Cup final, he shut down Jean Béliveau, the star centreman of the Montreal Canadiens, in the final two games and was voted the most valuable player of the playoffs, winning the Conn Smythe Trophy.
He remains the only Leaf ever to have won the trophy named for the former owner of the club. He was named team captain on October 31, 1969, succeeding George Armstrong who was said to be retiring from hockey. Armstrong returned to the Leafs two weeks later and played for another two seasons, but Keon remained captain and would wear the C through the rest of his years with the Leafs. Keon hoped to make Team Canada for the 1972 Summit Series, but was coming off one of the worst years of his career, finishing the 1971–72 season with his lowest points-per-game average since his rookie year. The final pick for Team Canada came down between Keon and Bobby Clarke. It is believed that Clarke was selected, as he had more points. He wasn't selected for Team Canada, but the Ottawa Nationals of the World Hockey Association made a strong effort to sign Keon, whom they had placed on their negotiation list earlier that year.
Harold Ballard, who had become the Leafs' majority owner in March 1972, said that Keon didn't provide the leadership the team needed during the previous season and was refusing to give Keon a big salary increase after a poor year. Keon signed a letter of intent with the Nationals, and received a $50,000 cheque from the team, but the deal fell apart just before training camp. Keon signed a three-year deal with the Leafs, and rebounded strongly in 1972–73, scoring 37 goals. On November 22, 1972 he scored his 297th goal as a Leaf, passing Armstrong and Frank Mahovlich to become the team's all-time leading goal scorer.
Early into the 1974–75 season, Ballard publicly blasted Keon, saying that the team wasn't getting good leadership from its captain and vowing never again to agree to a no-trade clause in a contract, as he had with Keon. When Keon's contract expired at the end of the season, Ballard made it clear that there was no place for him on the Leafs. The Leafs believed they had some strong young prospects at centre who needed more ice time, and Keon was again asking for a contract with a no-trade clause. The 35-year-old Keon was told he could make his own deal with another NHL team, but any club signing him would have been required to provide compensation to the Leafs. Ballard set the compensation price so high that other teams shied away from signing him, even though the Leafs had no intention of keeping him.
In effect, Ballard had blocked Keon from going to another NHL team. In August 1975, with the Leafs still controlling his NHL rights, Keon jumped to the World Hockey Association, signing a deal with the Minnesota Fighting Saints reportedly worth $300,000 over two seasons. The head coach of the Saints was Harry Neale, an old friend of Keon's. The team, and Keon, played well, but struggled badly financially. With 21 games left in the season, the team folded. Keon was expected to return to the NHL and wasn't included in the dispersal sale of Saints players to other WHA teams.
The NHL's New York Islanders wanted Keon, but needed to negotiate a deal for his NHL rights with the Leafs. Again, the Leafs' asking price (said to have been a first-round draft pick) was too high, and a disappointed Keon signed with the WHA's Indianapolis Racers in March 1976. The Fighting Saints were revived for the start of the WHA's 1976–77 season, and Keon was traded back to Minnesota, but the team folded for good half-way through the season (with Keon as its leading scorer).
Keon was briefly the property of the Edmonton Oilers who immediately traded him to the New England Whalers in January 1977. He would remain with the team through the rest of his career. In the 1977–78 season, Keon was joined on the Whalers by Gordie Howe, the team's leading scorer, despite turning 50 before the end of the season. Keon returned to the NHL in 1979 when the renamed Hartford Whalers became one of four WHA teams to join the NHL. Bobby Hull joined the Whalers that season, with Keon, Howe, and Hull sometimes playing as a forward line. Howe and Hull retired at the end of the season, leaving Keon as the oldest player in the NHL. Keon played two more seasons with the Whalers and announced his retirement on June 30, 1982 at age 42.