McKegg played four seasons of major junior hockey in the Ontario Hockey League, for a short time with the London Knights, but the majority with the Erie Otters, scoring 125 goals and 145 assists for 270 points, while earning 143 penalty minutes, in 262 games played. McKegg competed with Team Canada at the 2010 IIHF World U18 Championships, leading the team with six assists and seven points in six games played. On April 6, 2011, the Toronto Maple Leafs signed McKegg to a three-year entry level contract. On February 1, 2014, the Toronto Maple Leafs recalled McKegg from their American Hockey League affiliate, the Toronto Marlies. Taking Peter Holland's place on the roster, McKegg made his NHL debut that night playing three minutes and 43 seconds. He registered two hits, one shot on goal, and won one of his six face-offs in helping Toronto to a 6-3 win over the visitingOttawa Senators. On June 19, 2015, the Toronto Maple Leafs traded Greg McKegg to the Florida Panthers in exchange for Zach Hyman and a conditional 2017 draft pick. On September 25, 2015, was placed on waivers from Panthers to be assigned to affiliate, the Portland Pirates of the AHL. In the 2016–17 season, McKegg appeared in a career high 31 games with the Panthers for 6 points before he was claimed off waivers by the Tampa Bay Lightning on February 27, 2017. On July 1, 2017, McKegg was signed as a free agent to a one-year, two-way $650,000 contract to play for the Pittsburgh Penguins. He made an early impression in his new organization at training camp, making the opening night roster as the Penguins initial third-line center for the 2017–18 season. In 26 games, McKegg registered 2 goals and 4 points before he was placed on waivers by the Penguins, who returned to full health, on December 6, 2017. He was assigned to AHL affiliate, the Wilkes-Barre/Scranton Penguins, and contributed with 12 points in 28 games before on February 26, 2018, the Penguins traded McKegg to the Carolina Hurricanes in exchange for Josh Jooris. In the off-season, McKegg opted to continue with the Hurricanes, agreeing to a one-year, two-way contract on July 12, 2018. On July 1, 2019, McKegg was signed as a free agent to a one-year contract with the New York Rangers.
