Gene Conley as a pitcher with the Boston Red Sox in the early 1960s. (Sm/AP)

Gene Conley, a versatile athlete who had an unusual two-sport career in baseball and basketball and was the only player to win both a World Series title and an NBA championship, died July 4 at his home in Foxborough, Mass. He was 86.

He had heart ailments, said a daughter, Kelly Conley.

For six seasons in the 1950s and 1960s, the 6-foot-8 Mr. Conley went directly from baseball, where he was a three-time all-star pitcher, to basketball, where he became a key contributor to the Boston Celtics dynasty. He struck out baseball star Ted Williams in the 1959 All-Star Game and once scored over 7-foot-1 basketball super­star Wilt Chamberlain.

On the baseball mound, Mr. Conley was an intimidating right-hander with long limbs and a high leg kick. He was part of a Milwaukee Braves team that won the 1957 National League pennant and then defeated the New York Yankees in the World Series. His teammates included Hall of Famers Hank Aaron, Eddie Mathews and Warren Spahn.

In basketball, Mr. Conley won NBA championships in 1959, 1960 and 1961 with the Celtics, coached by Red Auerbach.

Boston Celtics forward Gene Conley, right, defending against the St. Louis Hawks’ Clyde Lovellette (34) in 1960. (AP/AP)

“He’s the most incredible athlete in the country,” Auerbach said of Mr. Conley in 1961. “He’s the only one who spends the entire year competing under the heaviest of pressure in two major sports. The average athlete would crack under that strain in a hurry. He thrives on it.”

Other players competed in both the NBA and major league baseball, including Chuck Connors (who became better known as an actor), Dick Groat, Dave DeBusschere, Ron Reed, Danny Ainge and Mark Hendrickson. Mr. Conley’s teammate on the 1957 Braves, catcher Del Rice, also played on the World Series-winning St. Louis Cardinals in 1946 and for the 1945-46 Rochester Royals, champions of the National Basketball League, three years before the National Basketball Association was formed.

Several pro football stars also played big league baseball, including Bo Jackson, Deion Sanders and Brian Jordan. Quarterback Otto Graham, who won three NFL championships with the Cleveland Browns, was also a member of the 1946 Rochester Royals.

But few athletes matched Mr. Conley’s long-term durability at the highest levels of two sports. He was often asked whether baseball or basketball was more demanding.

“Basketball, to me, was easier to play because it was an instinctive game,” he told the Miami Herald in 2008. “You can watch it, and you know instinctively what to do. Baseball is more of a thinking game.”

Mr. Conley first gained renown in baseball and was twice named minor league player of the year before he caught on with the Braves for good in 1954. He was the winning pitcher in the 1955 All- Star Game, coming on in relief to strike out Al Kaline, Mickey Vernon and Al Rosen in the 12th inning.

He appeared in 39 games for the Celtics during the 1952-1953 season before the Braves contractually prevented him from playing basketball for several years. In 1958, after a poor season on the mound, Mr. Conley feared his baseball career might be finished and signed with the Celtics.

As the backup to Hall of Fame center Bill Russell, Mr. Conley played infrequently but was a strong defender and rebounder who helped his team win three straight NBA championships.

“Had he concentrated on basketball earlier,” Hall of Fame teammate Tom Heinsohn said of Mr. Conley in 2008, “he would have been a really big-time, great player.”

In the meantime, Mr. Conley continued his baseball career with the Philadelphia Phillies in 1959 and 1960 before moving on to the Boston Red Sox.

In 1962, Mr. Conley was at the center of one of baseball’s strangest episodes. On a day in which he had given up eight runs to the Yankees, he was on the team bus when it was caught in a traffic jam. He asked if he and a teammate, Pumpsie Green, could get off the bus to use a nearby restroom.

By the time they returned, the bus was gone. Moreover, they missed the team flight to Washington. Mr. Conley and Green amused themselves for two days at New York watering holes before Green returned to the team.

For some reason, Mr. Conley bought an airline ticket to Israel — “I guess I thought they had weaker batters over there,” he said — but was turned away at the gate because he didn’t have a passport.

After three days, he went home to Massachusetts and was summoned to the office of Red Sox owner Tom Yawkey. He was fined $1,500 before being reinstated to the team.

“Mr. Yawkey offered me a drink right in his office at Fenway,” Mr. Conley told the Globe in 1992. “I said, ‘No thanks, Mr. Yawkey, never touch the stuff.’ ”

Donald Eugene Conley was born Nov. 10, 1930, in Muskogee, Okla., and was 11 when his family moved to Richland, Wash. His father worked at a power plant.

Mr. Conley could high-jump 6 feet 3 inches and played basketball and baseball at Washington State University before signing a minor league contract with the Braves.

He last pitched in the major leagues in 1963, with career record of 91-96. He continued to play in the NBA with the New York Knicks through the 1963-1964 season.

He settled in Massachusetts, where he ran an industrial paper supply business with his wife for more than 30 years. Known as a heavy drinker during his athletic career, he gave up alcohol in the 1960s and became a Seventh-day Adventist.

Survivors include his wife of 66 years, the former Kathryn Dizney of Foxborough; three children; a sister; seven grandchildren; and two great-granddaughters.

In 1964, Mr. Conley was hoping to reclaim his pitching form with a minor league club in North Carolina. But his chronically injured arm finally gave out on him, and he was released.

He went to a church on Mother’s Day and began to sob.

“All I’m thinking is, ‘I gotta get a job. I have three kids,’ ” he told the Orlando Sentinel in 2005. “This Southern deacon pokes me on the shoulder and says, ‘What’s the matter, son, did you lose your mother?’

“No,” Mr. Conley replied, “I lost my fastball.”

An earlier version of this story neglected to mention Del Rice, who was a member of the Rochester Royals’ basketball team, which won the National Basketball League title in 1946. He was also a member of the 1946 St. Louis Cardinals and the 1957 Milwaukee Braves, both of which won the World Series.