Killy, a descendant of an Irish mercenary soldier named Kelly who fought for Napoleon I, was reared at Val-d'Isère, a ski resort in the French Alps. In 1964 he became the leading French male skier, winning national championships in all three divisions of Alpine skiing: downhill (downhill skiing), slalom, and giant slalom. He was the European champion for the first time in 1965. In 1966 at Portillo, Chile, he won the world combined championship in the combination event.

In 1966–67 Killy won every downhill race he entered and earned the first World Cup for men (awarded by the Fédération Internationale de Ski 【FIS】 for finishing among the leaders most consistently in a series of international races). In that year and again in 1967–68, when he repeated his World Cup triumph, he led the French skiers to the world team championship. In the Olympic (Olympic Games) Winter Games of 1968 at Grenoble, France, he won gold medals for the downhill, slalom, and giant slalom races for men, becoming the second skier in Olympic history to sweep the Alpine events. (The first was Toni Sailer (Sailer, Anton) of Austria in 1956.)

After his retirement from competitive skiing in April 1968, Killy engaged in a number of activities, including acting and auto racing. He returned to skiing in 1972 as a professional, becoming world champion of professional skiers. Killy was a major organizer of the 1992 Winter Olympic (Olympic Games) Games at Albertville, France, and also served as an adviser or director for a number of corporations.