Shackleton began his career in the Yorkshire and Lancashire leagues as an all-rounder bowling some medium pace, but decided to concentrate on leg spin. He was also signed by Burnley F.C. as a goalkeeper. He joined the Army in 1942, entering the Pioneer Corps, and played Services cricket and football. He was signed by Hampshire in 1948, after being spotted by the Hampshire coach, Sam Staples. Originally destined to be a batsman, he was encouraged to return to pace bowling. In his first season, he did little of note, but in 1949, despite unfavourable pitches, Shackleton became the mainstay of Hampshire's attack. He took 100 wickets in 20 consecutive seasons, from 1949 until his retirement at the end of 1968. Only Wilfred Rhodes has taken 100 wickets in more seasons (23) but these were not consecutive. Shackleton almost reached a thousand runs as a batsman in his first season, but his batting subsequently declined until he was firmly entrenched in the lower order by the mid-1950s. He was awarded his county cap in 1949.