Conacher was born in Toronto, Ontario on May 24, 1900. His middle name was given after the South African city of Pretoria, where British troops were fighting the Boer War at the time of his birth. He was the eldest son of Benjamin and Elizabeth Conacher, and the third of ten children overall. He had four brothers and five sisters.

Rugby football was the first sport Conacher played, and it was his favourite. He first played organized football at the age of 12 as a middle wing with the Capitals in the Toronto Rugby Football League. He played four seasons with the team between 1912 and 1915, during which the Capitals won the city championship each year. He won the Ontario championship as a junior with the Toronto Central YMCA in 1918, and in 1919 moved up to the intermediate level. With the intermediate Capitals, he was moved into an offensive role as a halfback. He excelled in the role, and his team reached Ontario Rugby Football Union (ORFU) final. In that final, the Capitals' opponents from Sarnia made stopping Conacher their priority, a strategy that proved the difference as Sarnia won the championship.  Conacher moved to the senior level in 1920 with the Toronto Rugby Club where his team again won the ORFU championship, but lost the eastern semifinal to the Toronto Argonauts of the Interprovincial Rugby Football Union (IRFU). His play impressed the Argonauts, who signed him for the 1921 season. In his first game with the Argonauts, he scored 23 of the team's 27 points, and led the IRFU in scoring, accounting for 14 touchdowns and 90 of his team's 167 points as they went undefeated in six games. The Argonauts won the eastern championship, and faced the Edmonton Eskimos (renamed Edmonton 'Elks' in 1922) in the first east-west Grey Cup championship in Canadian history. Conacher rushed for 211 yards and scored 15 points in Toronto's 23-0 victory to claim the national title.  Named captain in 1922, Conacher led the Argonauts to another undefeated season in IRFU play, finishing with five wins and one tie, as he rushed for about 950 yards. The Argonauts reached the Eastern final, but lost to Queen's University, 12-11. In that game, Conacher was the entire Argonaut offense rushing 35 times for 227 yards but Pep Leadley's 21 yard field goal towards the end of the game gave Queens' its victory.

why did he leave?