Muhammad Ali winces as Ken Norton, left, hits him Sept. 10, 1973, in Inglewood, Calif. Six months earlier, they had boxed for the first time in what was expected to be an easy romp for Ali, but Norton surprised with a split-decision victory. (Anonymous/AP)

Ken Norton, one of the top boxers of the 1970s, who briefly held the heavyweight crown and was best known for breaking Muhammad Ali’s jaw in the first of their three memorable bouts, died Sept. 18 at a care facility in Henderson, Nev. He was 70.

His death was confirmed to the Associated Press by his son Ken Norton Jr., a coach with the Seattle Seahawks of the National Football League. The cause was congestive heart failure after a series of strokes in recent years.

Mr. Norton was a gifted all-around athlete who didn’t turn to boxing until he was serving in the Marine Corps in his 20s. Before he faced Ali in San Diego on March 31, 1973, he had been a lightly regarded journeyman who had fought few top-notch opponents. He was a single father raising a son on his own and made only $300 for his previous fight, which was held before a crowd of 700.

For his fight with Ali, he accepted $50,000 for a 12-round fight to be broadcast on national television. It was expected to be an easy romp for Ali, who was angling for a title shot against then-champion George Foreman.

The 6-foot-3, 210-pound Mr. Norton was a 5-to-1 underdog, but he was in superb condition and proved to be a formidable force in the ring.

“He had a body-builder’s physique, with arms that appeared ready to pop, the legs of a middleweight, a 44-inch chest and a 31-inch waist,” Sports Illustrated reported at the time. “No fighter Ali has met, save [Joe] Frazier” — the only boxer who had previously beaten Ali — “seemed less intimidated.”

Mr. Norton had an unorthodox style, with his arms often crossed in front of his body, that seemed to confuse Ali. In the second round, he caught Ali along the ropes and delivered a right uppercut. Most experts believe Ali’s jaw was broken with that punch, but he battled on gamely for 10 more rounds.

Mr. Norton won the fight by a split decision and immediately entered the front rank as one of the country’s leading heavyweights. It was only the second time Ali had lost in his professional career.

Before the fight, TV commentator Howard Cosell said it was one of the worst mismatches he had ever seen. Afterward, Cosell apologized, saying, “Kenny, you made me look silly.”

“That’s okay, Howard,” Mr. Norton replied. “You always look silly.”

Six months later, Mr. Norton narrowly lost a rematch to Ali, who went on to reclaim the heavyweight championship from Foreman in 1974. Mr. Norton met Ali for a third time on Sept. 28, 1976, at New York’s Yankee Stadium.

At the beginning of the 15th and final round, Mr. Norton believed he was ahead on points and would win the title if he could avoid being knocked out. Ali pressed forward during the round, as Mr. Norton waited until the final seconds to launch a counterattack. It came too late, as the judges awarded the fight to Ali in a close but unanimous decision.

“I look back and wish I could fight that last round over again,” Mr. Norton told Thomas Hauser for the 1991 oral biography “Muhammad Ali: His Life and Times.” “I won that fight . . . I’m getting mad again, talking about it. But Muhammad brought out the best in me. He was the best fighter I ever fought, and I respect him.”

By then, Mr. Norton had branched out to acting, with starring roles in “Mandingo” (1975) and “Drum” (1976), two films set on 19th-century plantations, and guest spots on TV series.

But his primary focus remained boxing, where he was reaping million-dollar paydays. In 1978, he became the heavyweight champion of the World Boxing Council, one of several sanctioning organizations, on a technicality.

The champion at the time, Leon Spinks, was required by WBC rules to face Mr. Norton, the top contender. But when Spinks opted to face Ali instead, the title was handed to Mr. Norton by default — making him the only heavyweight champion who never won a championship fight.

In his first defense, on June 9, 1978, Mr. Norton met undefeated challenger Larry Holmes in Las Vegas. After 14 rounds, all three judges had the same score on their cards: 133-133.

During the 15th and decisive round, the two fighters stood toe-to-toe in what historians consider one of the greatest title fights in history. Each fighter staggered the other with strong punches, but Holmes landed a powerful right just before the final bell.

Each judge had the same score for the fight, 143-142: two favoring Holmes, and one favoring Mr. Norton.

Holmes was so exhausted that he could barely lift his arms in victory.

Kenneth Howard Florence was born Aug. 9, 1943, in Jacksonville, Ill., and later took the name of his stepfather.

He was an athletic standout in high school and went to Missouri’s Truman State University on a football scholarship. He left to enter the Marine Corps, where he became the three-time heavyweight boxing champion. He turned professional in 1967.

Mr. Norton’s final bout came in 1981, when he suffered a merciless beating from Gerry Cooney. The referee stopped the fight after only 54 seconds. Mr. Norton’s career record was 42 wins, seven losses and one draw.

Survivors include his third wife, Rose Conant, and four children from his previous marriages to Jeanette Brinson and Jacqueline Halton. Complete information about survivors was not available.

His son Ken Norton Jr. played 13 years as an NFL linebacker. For several years in the 1990s, they were estranged before reconciling.

In later years, Mr. Norton served as a boxing commentator and entered various business ventures. He was named to several boxing halls of fame. After a serious car accident in 1986, he was in a coma and spent months in recovery. One of the first people he recognized when he regained consciousness was Ali, who had come to visit him in the hospital.

“I don’t want to be remembered as the man who broke Muhammad Ali’s jaw,” Mr. Norton told Hauser for Ali’s biography. “I just want to be remembered as a man who fought three close competitive fights with Ali, and became his friend when the fighting was over.”