William is the nephew of John and Michael Whitaker and cousin of Robert, Louise and Ellen. His father Ian is the only non-show jumping brother of the Whitaker family.

William has been around horses all his life and started competitive career in 1997, aged eight. In 2001, he won the Horse of the Year Show 138cm Championship on Mystic Starlight Express and in 2003 went on to win the Junior Foxhunter on Devlin. Two years later he won the Junior Show Jumper of the Year class riding Indiene de Here and was also second on Colton Maelstrom.

On leaving school, William went to Florida for three months to train with top US show jumper Aaron Vale, who helped him to make the changes necessary to move from from riding ponies to horses. After his time with Aaron, William returned to the UK to work for Michael Whitaker, his uncle. In August 2010, he joined Shaw Farm Stud to work alongside Lisa Hales, the owner of Arko III and Russel II.