Angelo Dundee, a boxing trainer of matchless cunning who guided Muhammad Ali’s career for two decades and who worked in the corner with Sugar Ray Leonard and more than a dozen other world champions, died Feb. 1 in Clearwater, Fla., after a heart attack. He was 90.

His son, Jim Dundee, said his father died at a rehabilitation center in Clearwater. He had been in Louisville two weeks earlier to commemorate Ali’s 70th birthday but was hospitalized for a blood clot after his return to Florida.

Mr. Dundee worked in boxing for more than 60 years but was best known for molding the young Ali into a champion.

They met in the late 1950s, when Ali — then a teenager known as Cassius Clay — called on Mr. Dundee for boxing advice. He became Ali’s full-time trainer after the boxer won an Olympic gold medal in 1960.

Less than four years later, on Feb. 25, 1964, Ali scored a shocking upset by beating Sonny Liston to win the world heavyweight championship. He went on to become one of the greatest boxers of all time.

“If Angelo hadn’t been in my corner, I wouldn’t be where I am today,” Ali told the New York Times in 1981. “I might have made it to the top, but not as quickly. As a corner man, Angelo is the best in the world.”

Boxing historians agree that Mr. Dundee’s tactical and psychological guidance proved essential to Ali’s success. Working out of the run-down Fifth Street Gym in Miami Beach, Mr. Dundee quickly assessed Ali’s strengths as a boxer. Even more important, he developed a keen understanding of his psychology and knew when to praise Ali and when to prod him.

Other trainers had tried to remake Ali’s unconventional yet graceful style in the ring, but Mr. Dundee had the good sense to leave him alone. When Ali joined the Nation of Islam, Mr. Dundee never raised an objection. He made it a point not to interfere in his boxers’ religious or romantic lives, preferring to focus his attention on what happened inside the ropes.

Mr. Dundee, who began his career carrying buckets, taping fighters’ hands and stopping their bleeding, knew all the tricks to help his proteges out of tight spots. In 1963, when Ali was knocked down in a fight with British boxer Henry Cooper, Mr. Dundee drew the referee’s attention to a torn glove on Ali’s hand.

Whether the rip was already present or whether Mr. Dundee cut the glove to delay the fight has been a matter of speculation for almost 50 years. At any rate, Ali recovered his strength and knocked Cooper out in the next round.

Mr. Dundee supported Ali’s penchant for showmanship, recognizing it as a central part of the boxer’s appeal. He sometimes helped Ali write the doggerel for which he became famous, including this rhyme before the 1964 title fight with Liston:

Who would have thought when they came to the fight?

That they’d witness the launching of a human satellite.

Yes the crowd did not dream, when they put up the money,

That they would see a total eclipse of the Sonny.

During the fight, in which Liston was heavily favored, Mr. Dundee proved himself as a master tactician. After the fourth round, Ali returned to the corner blinking from a substance in his eyes.

“Cut the gloves off,” he said. “I can’t see!”

“I put my pinkie in his eye and put it in my eye,” Mr. Dundee told this reporter for the 1999 book “Muhammad Ali: The Miami Years, 1961-1964.” “It burned. Definitely a caustic substance.”

Mr. Dundee splashed water in Ali’s eyes and, as the bell rang for the fifth round, pushed his fighter into the ring, telling him to keep on the move, out of Liston’s range.

“You can’t quit now,” Mr. Dundee said. “This is the big one, Daddy! Run!”

By the end of round, Ali’s eyes began to clear. In the sixth round, he was back on the attack, opening a deep cut under Liston’s eye. Beaten and demoralized, Liston did not answer the bell for the seventh round, relinquishing his title.

The brash, young Ali climbed on the ropes of the ring, shouting, “I am the king! King of the world! I am the greatest.”

Angelo Mirena Jr. was born Aug. 30, 1921, in Philadelphia. His father was a railroad worker, and his Italian mother never learned English. As a young man, Angelo changed his last name to Dundee, as did two of his older brothers who were involved in boxing.

Mr. Dundee served in the Army Air Forces during World War II, then moved to New York in 1948 to apprentice in the fight game. He worked at Stillman’s Gym, a hotbed of boxing, under noted trainers Ray Arcel, Chickie Ferrara and Charley Goldman.

By 1952, Mr. Dundee and his brother Chris, a promoter, were establishing a boxing stronghold in Miami Beach. The first fighter Mr. Dundee took to a championship was Carmen Basilio, who won the welterweight title in 1955 and became middleweight champion, defeating Sugar Ray Robinson.

His other titleholders over the years included Willie Pastrano, Sugar Ramos, Luis Rodriguez, Ralph Dupas, Jose Napoles, Wilfredo Gomez, Michael Nunn, Jimmy Ellis and the 45-year-old George Foreman, when he became heavyweight champion in 1994.

Mr. Dundee began working with Sugar Ray Leonard in the 1970s and helped lead the Washington area boxer to one of his greatest victories. Leonard had battered Tommy Hearns in the early rounds of a 1981 welterweight title bout, but Hearns fought back.

After the 12th round, Leonard’s eye was badly swollen, and he was in danger of losing the fight. Mr. Dundee stood inches from his face and shouted: “You’re blowing it, son. You’re blowing it! This is what separates the men from the boys. You’re blowing it!”

Leonard knocked Hearns down twice in the 13th round and won by technical knockout in the 14th.

“Angelo just knew what to do,” boxing historian Bert Sugar, who collaborated with Mr. Dundee on an autobiography, said Thursday. “In any grouping of the great trainers in history, Angelo would be front and center in the front row.”

Mr. Dundee was named to the International Boxing Hall of Fame in 1992 and remained an active trainer until his death.

His wife of 59 years, Helen Bolton Dundee, died in 2010.

Survivors include two children, Jim Dundee of Dunedin, Fla., and Terri Dundee Coughlin of Land O’Lakes, Fla.; six grandchildren; and a great-grandson.

In 1964, when Pastrano was in danger of losing his light-heavyweight title, Mr. Dundee grabbed the boxer’s face and swatted him hard on the rear.

Pastrano almost took a swing at Mr. Dundee, who said, while gesturing across the ring, “Hit him, not me, you S.O.B.”

Pastrano knocked out his opponent in the next round.