He won the 1956 Canadian Open Chess Championship. In 1957, Lombardy became the first American to win the World Junior Chess Championship and, in so doing, the first American World Chess champion of any kind. He won the tournament in Toronto with a perfect score of 11-0, the only time such a result has been achieved.

Lombardy played first board for the US Team that won the 1960 World Student Team Championship in Leningrad, USSR, the first time the US Team won that event (they would win again at Haifa 1970) (http://www.olimpbase.org). Lombardy defeated future World Chess Champion Boris Spassky in their individual game. Lombardy won a gold medal for best result on his board in that event.

Lombardy represented his country in several Chess Olympiads, and played many times in the US Chess Championship. Lombardy finished second in the 1960-61 US Chess Championship behind Bobby Fischer and ahead of Raymond Weinstein and a star-studded field. With this result, Lombardy qualified to compete in the World Interzonal tournament in Stockholm for the World Chess Championship. However, Lombardy decided instead to retire from chess to become a Catholic priest.

In 1972, Bobby Fischer was scheduled to play a match against Boris Spassky for the World Chess Championship. However, Fischer had a falling out with Grandmaster Larry Evans who had been Fischer's second in his successful matches against Tigran Petrosian and Bent Larsen. Fischer called upon his old friend William Lombardy to help him out with the match. Although Lombardy was still a Catholic priest, he was allowed to take time off from the priesthood to go to Reykjavík, Iceland to serve as the official "second" to Fischer during the World Chess Championship 1972, between Fischer and Boris Spassky. Nevertheless Fischer often did not make use of his services. Don Schultz remembers the following conversation: Lombardy to Fischer 'That's a difficult position. Let's go back to the hotel and analyze it.' Fischer replied 'What do you mean, analyze? That guy's a fish. Let's go bowling.' According to Lombardy, this story is inaccurate. Fischer won the match and became World Chess Champion.

Lombardy later left the priesthood. He is now retired and lives in the East Village of New York City, where he is writing a chess book and sells chess lessons at his home and by appointment elsewhere.