In 1910, Thys won Belgium's first national cyclo-cross championship. The following year he won the Circuit Français Peugeot, followed by stage races from Paris to Toulouse and Paris to Turin. He then turned professional to ride the Tour de France. Thys won the Tour in 1913despitebreaking his bicycle fork, and needing to find a bicycle shop to mend it. The repair induced a 10-minute penalty, but he won with a lead of just under nine minutes. Thys took the stage and the race lead when Eugène Christophebroke his fork on the way to Luchon. Marcel Buysse overtook him in the results the following day. Another broken fork on the way to Nice gave Thys the lead again but drama continued when he fell on the penultimate stage from Longwy to Dunkirk. Despite being knocked out and being penalised for help from teammates to repair his bike, he won 8 minutes and 37 seconds ahead of Gustave Garrigou, with Buysse third. In 1914, he took his first stage victory, to Le Havre, holding the race from start to finish despite a 30-minute penalty for an unauthorised wheel change on the penultimate stage. His victory looked uncertain, his lead cut to less than two minutes ahead of Henri Pélissier. Ironically, on the final stage from Dunkirk to Paris, the Frenchman's supporters along the route who were expecting victory over the Belgian were the reason he was prevented from launching a breakaway. He won the stage but Thys finished on his wheel to win the Tour. In 1917, Thys won Paris–Tours and the Giro di Lombardia. In 1918, he also won the second and last Tours–Paris. After World War I, Thys won the Tour a third and final time in 1920. He led from the second stage, Henri Desgrange writing "France is not unaware that, without the war, the crack rider from Anderlecht would be celebrating not his third Tour, but his fifth or sixth". Not until 1955 did Louison Bobet equal Thys's record, and not until 1963 did Jacques Anquetil break it with four wins. Thys also rode in the 1922 Tour, winning five stages, and in the 1924 Tour, winning two stages. Thys was one of a generation of cyclists whose careers were disrupted by the First World War. After retiring, he recalled that he had been asked by his manager, Alphonse Baugé, to wear a yellow jersey as leader of the Tour, although that distinction is more commonly attributed to Eugène Christophe.
