Shackleton signed amateur forms with Bradford Park Avenue after being signed by manager Billy Hardy. Bradford PA permitted him to play for Kippax United in the Leeds League. Arsenal's secretary-manager George Allison heard of Shackleton's talents, and drove to Bradford to sign him to amateur forms in 1938. Arsenal permitted him to play for Enfield in the Athenian League, and he in fact only represented Arsenal twice in reserve team fixtures in the Southern League. He was released by Arsenal in May 1939 and told by Allison that he was too small to succeed as a footballer, and should find other employment. Following this disappointment he took a job at London Paper Mills in Dartford, and turned out for the factory's works team in the Kent League. Bradford Park Avenue Shackleton returned to his hometown upon hearing of the outbreak of World War II and took up employment assembling aircraft radios for GEC, at which point he rejoined Bradford Park Avenue as an amateur after being invited to the Park Avenue Stadium by manager David Steele. He turned professional at the club shortly before Christmas 1940 and received a £10 signing-on fee, which the cash-strapped club had to pay in instalments. On Christmas morning he played for Bradford PA, then guested for Huddersfield Town in the afternoon, and scored in both matches. In the wartime leagues he scored a total of 171 goals in league and cup 209 appearances for Bradford PA. He became a Bevin Boy in order to avoid his call-up for national service in 1945 as he did not want to miss the resumption of the Football League, but found the experience of coal mining terrifying and gruelling. He began to avoid his shifts in the pits, and so was called into the Royal Air Force, where he served the remainder of his national service. He scored four goals in seven Second Division matches at the start of the 1946–47 season, but left the club in October 1946 after growing tired of heckling from his own supporters who did not appreciate his individualist style. Newcastle United In October 1946, Shackleton was sold to Second Division side Newcastle United for a £13,000 fee. He was sold as a direct replacement for Albert Stubbins, who had been sold from Newcastle to Liverpool for the same fee. He scored six goals on his debut in Newcastle's 13–0 defeat of Newport