Born in Sydney on 7 November 1944, Windle grew up in the southern Sydney suburb of Waverley. Windle and his sister Norma learnt to swim at nearby Bronte Beach. His family later relocated to Bexley North, and he swam for his school team at Marist College Kogarah. Windle was coached by Frank Guthrie at the Enfield pool from the age of 12. After a fortnight of training, he entered the district age group championships and won a place in the top three. From then on, he began training three or four times a week, and his father drove him to Enfield for morning training. By the age of 15, he was training everyday.
A turning point for Windle was the 1960 Australian Championships; he came second in the 1650 yd freestyle behind John Konrads and was selected for the 1960 Summer Olympics in Rome. He dropped out of high school to attend a three-month training camp in Queensland with the Olympic swimming team. Windle did not swim in Rome; team officials merely wanted him to learn from the routine and atmosphere of Olympic swimming. On his return to Australia, he switched to the tutelage of Don Talbot, who also coached Konrads.\\\