Girard won the Stanley Cup four times as a player during a 10-year pro career spent almost entirely with his hometown Ottawa Senators. He also coached the Montreal Maroons to the Cup in 1926 and was among the nine players inducted into the Hockey Hall of Fame when it was founded in 1945.

The Senators, then in the National Hockey Association, tried to get Girard to turn pro as early as 1910, but it wasn't until early 1914 that he did so. Girard was an instant hit, scoring six goals and finishing with 13 points in 11 games. He helped Ottawa advance to the Stanley Cup Final in 1915, although the Senators lost the best-of-five series to Vancouver of the Pacific Coast Hockey Association, and scored 13 goals in 24 games in 1915-16 and 18 in 19 games the following season.

Ottawa joined the NHL for the 1917-18 season, and Girard was a key member of the Senators teams that won the Stanley Cup three times in four seasons from 1919-23. By then, Girard had switched from forward to defenseman, though he continued to produce offensively. The Senators, who won the championship in 1920 and 1921, had their bid for a third straight Stanley Cup title ended by the Toronto St. Patricks in the 1922 NHL championship series. However, when Toronto defenseman Harry Cameron was injured in the Cup Final against Vancouver, Girard was allowed to join the St. Patricks as a replacement; he played a key role in helping them win Game 4, forcing the best-of-5 series to the limit. Cameron returned for the deciding Game 5, a 5-1 victory, and Girard was included on the winning roster.

Girard returned to the Senators and helped them win the Cup again in 1923, giving him four consecutive championships. However, a throat ailment stemming from when he was hit by a shot forced him to retire and left him with a weak voice for the rest of his life. He finished with 101 points (50 goals, 51 assists) in 128 NHL games.

The Maroons hired Girard as general manager and coach during the 1924-25 season. They were 1-10-0 in 11 games after the coaching change, but went 20-11 with five ties in 1925-26, then defeated Victoria of the Western Hockey League to win the Cup. The Maroons made it back to the Cup Final in 1928 but lost the best-of-5 series to the New York Rangers in five games.

Girard coached the Maroons through 1928-29 before resigning, spent two seasons running the New York Americans, then returned to the Maroons for two seasons. The Senators relocated for the 1934-35 season and were renamed the St. Louis Eagles; they hired Girard as general manager and coach. After a 2-11-0 start and with his throat problem still an issue, Girard resigned on Dec. 11, 1934 and died of throat cancer on Aug. 7, 1937 at the age of 47.