Born in Ottawa, Ontario, Canada, Sixsmith learned ice hockey at an early age. He played in 1901 for the Canadian Soo. That same year his brother Arthur, visited Pittsburgh, and decided to stay, and help found a hockey league. Art then convinced Garnet and several other Canadian players to play in the very first openly professional league, the Western Pennsylvania Hockey League. He then moved to Pittsburgh to play professionally in 1902. Sixsmith would play professionally for several WPHL teams, the Pittsburgh Victorias, Pittsburgh Lyceum and Pittsburgh Athletic Club between 1902 and 1910. He also played in the International Professional Hockey League for the Pittsburgh Professionals. Garnet spent his entire professional career in Pittsburgh, with the exception of one season in which he played for the Canadian Soo. In one memorable game, the visitingPortage Lakes Hockey Club played at the Duquesne Gardens. Just before the game began, Portage Lakes' William "Lady" Taylor told Garnet; "I'm going to break your leg tonight". Sixsmith replied to Taylor with the Bronx cheer gesture. However, later in the game, Garnet's leg was broken, in three separate locations. Of the incident, Garnet later stated that he was shocked that any man could hurt him like that because he was always able to take care of himself. However at Sixsmith's very next time game at the Duquesne Gardens, he scored 11 goals in that game. The 11 goals is considered to be a record for the Duquesne Gardens, one that has not yet been broken by any Pittsburgh hockey player. In 1904, Sixsmith became the first hockey player to use aluminum skates, after he noticed that they were used by speed skaters. Wanting more speed in his game, Garnet then had aluminum ice skates custom-made with a shorter blade, for himself. The skates costs $15, even though his brother and several others told him they would never become popular. The skates eventually helped him earn his reputation as a fast skater. The type skate developed for Garnet soon became used by the skate of choice for hockey players and are still in use. The original skates were later gold plated by his family and given to him on February 3, 1960, his 50th wedding anniversary. He joined the Pennsylvania Railroad as a clerk in 1905. After hockey, he continued at the railroad, eventually becoming a superintendent of the Conemaugh Division. In 1915, Sixsmith turned down an offer to coach the Princeton Tigers hockey team. That same year, he played on a team for the Pittsburgh Winter Garden alongside his brother, who was also the team's manager. The team lasted on one year and after the season both brothers ended their playing careers. At this time six-man hockey came to Pittsburgh and the new game did not appeal to Sixsmith. He later stated that the seven-man game "was tougher and had more action". Garnet's last connection to hockey came in 1922, when he served as the president of the amateur Pittsburgh Yellow Jackets. On November 16, 1935, Sixsmith dropped the ceremonial first puck, at the Duquesne Gardens, for the inaugural home game of the Pittsburgh Shamrocks of the International Hockey League He retired in 1950 and moved to Chautauqua, New York. He became critical of several of the changes to the game. In 1933, he stated to the Pittsburgh Press he couldn't bear watching a team score a goal and then drop back and play defensive hockey. Stating that the best defense was the best offense, Sixsmith felt that teams should keep scoring and pile up the score. He also stated that the referees had too much responsibility and blew the whistle too often. Finally he stated that line changes were occurring to often. Sixsmith felt that pulling a player after being on the ice for 3–4 minutes was unfair since he felt it took that long for a player to warm-up.
