His career started at Arsenal where he was signed as an amateur and was employed as a member of the club's ground staff during the summer of 1938. He made one appearance for Arsenal's A team in the Southern League, scoring one goal. However, on the outbreak of World War II in September 1939 he was released by the club.
He came to the attention of the talent scouts by scoring 166 goals in six years of wartime football at Bradford Park Avenue. In February, 1948, after less than two years with Newcastle United F.C., where he was transferred for £13,000, he was signed to Sunderland for a then-record fee of £20,500. Shackelton was adored by Sunderland fans and he was known to prefer Sunderland to his previous club, local rivals, Newcastle, once remarking: "I'm not biased when it comes to Newcastle - I don't care who beats them!"
Journalist Malcolm Hartley, who wrote a history of the Bradford team, wrote of Shackleton: "Apart from the adhesive ball control and breathtaking body swerve, Shack could hit a ball. His slender legs could crack the ball like a Bofors gun." His name is still a byword for mastery in football.
Shack, as he was known, scored 101 goals in 348 games with Sunderland, but his team's fortunes were poor and despite his great work and astounding ball skills he never reached a cup final, though he did make two semi-finals with Sunderland.
He gained 5 international caps for England during his career. England boss Walter Winterbottom would tell of his unsuccessful attempts to 'tame' him for the England set up - "If only Len would come half-way to meet the needs of the team there wouldn't be many to touch him." Shack shrugged off his country's continued snubbing, saying "they play at Wembley stadium, not the London Palladium.
He was forced into retirement in September, 1957, by an ankle injury, and became a journalist.
Shackleton was famous both on and off the field for his sense of humour. He used his nickname, "The Clown Prince of Football", in 1956 for his autobiography. One chapter of that book was The Average Director's Knowledge of Football. It consisted of a single blank page.
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