William was the fourth son in a family of thirteen and weighed 14 pounds (6.35 kg) at birth. 

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His father, a master tailor measured 5 ft 9 in (1.75 m), while his siblings and mother Anne were of average size, although one sister who died in an accident at age 16 was tall.

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He was said to have been teased at school because of his height, though many students were scared of him.  Teachers at the school were said to have punished misbehaving students by getting Bradley to lift them onto high cross beams, until the teacher decided to have them taken down again. After leaving school he worked on a farm near the town of Pocklington, earning less than 10 shillings (50p) a week.

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​Bradley travelled with a group of showmen under the alias of the Yorkshire Giant; at the time, the freak shows were popular and would draw large crowds. As the tallest British man, Bradley was a prized asset in the business joining the huge Yorkshire Pig which was bred in Sancton two miles from Market Weighton.

​After touring many fairs up and down the country, including the Hull Fair, he parted from his minder by 1815 to manage himself. He would charge a shilling for each person to visit him in a room which he hired in various towns.He was even presented before King George III at Windsor who gave him a large golden watch on a chain, which he wore for the rest of his life.

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Bradley died aged 33 in his hometown of Market Weighton, where he was buried inside the church because of fears of graverobbing. His house still stands on Market Hill. The A1079 road which passes Market Weighton is named Giant Bradley Way.