Williams surprised many when he won the Canadian trials in record time and was sent out to the 1928 Olympics in Amsterdam at age 20. His volunteer coach, Bob Granger, had earned his fare to the Canadian championships in eastern Canada by working on the train serving food in exchange for passage. Vancouver track fans raised the money to pay Granger's transatlantic ship passage to the Olympics. To his surprise, Williams found out that he could easily advance to the final of the 100 m event. A good start in the final gave Williams the early advantage to win the race. He repeated his performance in the 200 m to come home with two gold medals—cheered by thousands of enthusiastic Canadians. He was also a member of the Canadian team which was disqualified in the final of the 4×100 metre relay contest.
He showed that his success was not an accident, winning the 100 yard dash at the inaugural British Empire Games (now known as the Commonwealth Games) in Hamilton, Ontario, Canada, and setting a World Record at a meet in Toronto in 1930. He suffered a pulled thigh muscle at the British Empire Games and never made a full comeback. At the 1932 Summer Olympics in Los Angeles, he was eliminated in the semi-finals of the 100 metre event. With the Canadian team he finished fourth in the 4×100 metre relay competition. Subsequently, Williams stopped running and became an insurance agent.
Williams lived with his mother until she died in 1977. After that, he lived alone and suffered in constant arthritic pain. Percy Williams was a great collector of guns and committed suicide by shooting himself in the head, ironically with a gun he had been awarded in 1928 as a prize for his Olympic feat. No note was left; it was a surprise to everyone. He was interred at Masonic Cemetery of British Columbia, Burnaby, Canada.
In 1950, a Canadian press poll proclaimed Percy Williams Canada's greatest track athlete of the first half of the century. They updated that in 1972 to declare him Canada's all time greatest Olympic athlete. He had donated in 2 gold medals from the 1928 Olympics to the BC Sports Hall of Fame, saying that he wanted them to be seen and remembered. Within weeks they were stolen. It was said at the time that Percy Williams, the fastest man of his generation, simply shrugged off the loss and no replacements were ever issued