Just two weeks after helping the Boston Celtics clinch the National Basketball Association championship in April 1961 with a 121-112 victory over the St. Louis Hawks, Gene Conley performed his annual rite of spring.

The only professional athlete to play for Major League Baseball and NBA championship teams, Mr. Conley swapped his No. 17 Celtics jersey for his No. 18 Red Sox uniform. He pitched eight scoreless innings in a 6-1 victory against the Washington Senators at chilly Fenway Park with Celtics teammates Bill Russell and K.C. Jones in the stands.

A three-time National League All-Star, Mr. Conley was also the only athlete to play for three professional teams in the same city: the National League’s Boston Braves, the Red Sox, and the Celtics.

Such accomplishments drew admiration, and some ribbing, too. Mr. Conley told the Los Angeles Times in 2008 that Celtics coach Red Auerbach once chided him: “ ‘Well, Gene, the playoffs are over, the season’s over, now you can go down and try to get out of shape so you can pitch.’ He thought baseball was a sissy game, I think.’’

Mr. Conley, who pitched for the 1957 World Series champion Milwaukee Braves and won NBA championship rings with the 1959, ’60, and ’61 Celtics, died of congestive heart failure July 4 in his Foxborough home. He was 86.

As the National League’s winning pitcher in the 1955 All-Star Game in Milwaukee, Mr. Conley struck out American Leaguers Al Kaline, Mickey Vernon, and Al Rosen in the top of the 12th inning.

Mr. Conley was “one of the nicest guys you could ever meet,’’ said Don Schwall, a former Red Sox teammate and pitcher. “He pitched the final inning of the first game I ever won at Fenway Park, gave me the game ball, and we were friends from then on.’’

A 6-foot-8, 225-pound right-hander, Mr. Conley was a powerful presence in both of his sports, which Auerbach put to good use.

“He was strong and physical,’’ Schwall said, “and he used to tell me stories about how Red used him as an enforcer at center when Bill Russell needed a rest.’’

On the diamond, Mr. Conley once decked the fiery Billy Martin and broke his jaw during a game in Cincinnati. “Boy, I let him have it. . . . He did a full gainer,’’ Mr. Conley told the Globe in 2008, adding: “He turned around and said, ‘Conley, I’m going to get a stepladder and get you next time.’ ’’

Mr. Conley posted a 91-96 record and 3.82 earned run average in 11 Major League seasons.

He broke in with the Boston Braves in 1952 and also played for the Celtics in 1952-1953 before deciding to focus strictly on baseball, playing for the Philadelphia Phillies before being traded to the Red Sox. He returned to two-sport playing in 1958, joining the Celtics as a rugged rebounder and low post defender. His final two NBA seasons were with the New York Knicks.

“He could really get up and down the floor,’’ recalled Celtics teammate Tommy Heinsohn, Mr. Conley’s roommate on road trips. “Two sports worked for him because he was such a great athlete, and because if you played for the Celtics, you’d better be in shape. He was a fun guy who looked at the brighter side of life, and if he had concentrated on basketball earlier he would have been a great pro player.’’

One hot July 1962 day in New York City, when Mr. Conley was pitching for the Red Sox and got shelled by the Yankees, he wasn’t looking at life so positively. When the team bus was caught in Manhattan traffic en route to the airport, Mr. Conley and infielder Pumpsie Green walked away, claiming they were looking for a bathroom. “I pulled Pumpsie aside and said, ‘Pumpsie, let’s get off this bus. Let’s go inside and have a cold one,’ ’’ Mr. Conley told the Globe in 1993. When they returned, “Pumpsie said, ‘Hey, that bus is gone,’ and I said, ‘We are, too.’ And we were off. And we were gone for three days.’’

Green returned to the team first, but Mr. Conley spent three days AWOL, living it up. The Red Sox fined him $1,500, though owner Tom Yawkey returned the money after Mr. Conley bounced back with a 15-win season for the eighth-place Sox. Mr. Conley gave up drinking after his playing days were over.

Second baseman Chuck Schilling said Mr. Conley was “kind of down on himself that day, and that was rare for him. He was probably tired of losing. To me, he was a special person and a special athlete who made friends very easily.’’

A son of Raymond Conley and the former Eva Brewer, Donald Eugene Conley was born in Muskogee, Okla., and graduated from high school in Richland, Wash., where he starred in basketball and was on the baseball and track teams in the spring, sometimes performing for both teams the same day.

A baseball and basketball star at Washington State University, Mr. Conley signed with the Boston Braves after his sophomore season.

He married Kathryn Dizney in 1951. Together, they ran Foxboro Paper Co., an industrial packaging supply business, for 35 years.

Mr. Conley and his wife, along with Celtics star Bob Cousy and a few others, successfully lobbied the NBA to initiate a revised pension plan for players who played before 1965.

In 2004, Mr. Conley’s wife, who is known as Katie, completed an eight-year writing project when her book “One of a Kind’’ was published. It tracks Mr. Conley’s unique career and personal life, chronicling his adventures, mishaps, and triumphs on and off the court and the diamond.

“His opponents respected him and so did his Celtics teammates,’’ longtime friend Satch Sanders wrote in an endorsement for the book. “Being a Celtic is being a fraternity of brothers.’’

Giving up sports wasn’t easy. On Mother’s Day 1964, Mr. Conley sat in a North Carolina church crying because he knew shoulder problems had ended his pitching career. “This southern deacon pokes me on the shoulder and says, ‘What’s the matter son, did you lose your mother?’ ’’ Mr. Conley recalled in a 2005 interview with the Orlando Sentinel. “No,’’ he told the deacon, “I lost my fastball.’’

In addition to his wife, Mr. Conley leaves his son, Dr. Gene R. Conley of Walla Walla, Wash.; his daughters, Dr. Kitty Quick of Norfolk and Kelly Conley of Norton; his sister, Billye Lynn Drew of Seattle; seven grandchildren; and a great-granddaughter.

A funeral service and burial will be private.

“I have always been struck by what made dad so humble about his days in sports,’’ Kitty said. “He knew he wasn’t a Bill Russell or a Hank Aaron. But he was a superstar as a human being, a husband, a father, and a mentor, and he was always approachable.

“He would say, ‘Red would never have you on the team if you couldn’t contribute,’ and he truly loved his teammates. His face would light up whenever one of them called.’’

Marvin Pave can be reached at marvin.pave@rcn.com.