Walker was born in Hobart, Tasmania, on 12 September 1948. While in high school he represented Tasmania as an opening batsman in their colts team, scoring a century in one match. After graduating from a Hobart high school in 1967 he relocated to Melbourne to play football and also enrolled in an architecture degree at the Royal Melbourne Institute of Technology. He graduated with a fellowship diploma in 1973.

In 1966, while still a high school student, Melbourne Football Club coach Norm Smith visited Walker's family home in Hobart to sign him to the Demons. He relocated from Hobart to Melbourne in 1967 and made his VFL debut the same year. He spent six seasons with Melbourne, playing 85 games as a ruckman and defender and earning one Brownlow Medal vote in 1968. After returning from the Australian cricket team's 1972–73 tour of the West Indies, Walker informed the club that he would not be joining them for the 1973 VFL season, as the season was about to start and he needed to complete a thesis for his university degree.

Upon moving to Melbourne in 1967 Walker joined the Melbourne Cricket Club. Although he earned a reputation as a batsman as a junior, he was given an opportunity to open the bowling attack for Melbourne, which showed his true cricket potential. His strange bowling action, bowling right arm over his right leg, earned Walker the nickname of "Tanglefoot" which was quickly shortened to "Tangles". He made his first-class debut for Victoria in February 1969 against Queensland, the final match of Victoria's 1968–1969 Sheffield Shield season. Despite taking five wickets in the match, he did not get a chance to play in the 1969–1970 season due to the depth in Victoria's bowling. He earned a second match in December 1970 and became a regular in the 1971–72 season, playing a total of 135 matches for Victoria.

He made his test debut for Australia in the second Test against Pakistan that commenced on 29 December 1972. He took five wickets in the match and retained his place in the team for the next match, the final of the series. In that match, a final innings performance of 6–15 by Walker prevented Pakistan from winning the match. During the series he was selected for Australia's tour of the West Indies that ran from February to April 1973.

Walker played a total of 34 Tests, taking 138 wickets as a medium-fast bowler. He played in 17 ODI's between 1974 and 1981, as well as playing in the breakaway World Series Cricket from 1977 to 1979. His final m