Norton was an outstanding athlete at Jacksonville High School. He was selected to the all-state team Football team on defense as a senior in 1960. His track coach entered him in eight events, and Norton placed first in seven of them. As a result, the "Ken Norton Rule", which limits participation of an athlete to a maximum of four track and field events, was instituted in Illinois high school Sports. After graduating from high school, Norton went to Northeast Missouri State University (now Truman State University) on a football scholarship and studied elementary education. In an interview with ESPN Fitness Magazine in 1985, Norton said that he would have become a Teacher or a policeman if he had not taken up boxing.

Norton started boxing when he was in the United States Marine Corps from 1963 to 1967, compiling a 24–2 record en route to three All-Marine Heavyweight titles. In time, he became the best boxer to ever fight for the Marines, and was awarded the North Carolina AAU Golden Gloves, International AAU and Pan American titles. Following the National AAU finals in 1967, he turned professional.

Norton was married three times and had four children. His first marriage (1966–1968), while still in the Marines, was to Jeannette Henderson, and produced football player and coach Ken Norton Jr.. In 1977 he married Jacqueline 'Jacquie' Halton, and birthed to Sports anchor Keith Norton, daughter Kenisha Eronda (1976) and son Kene Jon (1981) before Jacquie and Ken divorced in 2000. Around 2012 he married Rose Marie Conant who was his constant companion until his death.

'Name' opponents were elusive in Norton's early career. His first big break came with a clear win over respected contender Henry Clark. This helped get him his world recognition break when Ali agreed to a match. Joe Frazier, who'd sparred with Norton, presciently said of Ali, "He'll have plenty of trouble!" Though both were top boxers in the mid 1970s, Norton and Frazier never fought each other, in part because they shared the same trainer, Eddie Futch, and also that they were friends.

Norton, a proponent of motivational author Napoleon Hill's writings (e.g. Think and Grow Rich as noted above and Success Through A Positive Mental Attitude by Hill and W. Clement Stone) also received the "Napoleon Hill Award" for positive thinking in 1973.

In 1974, Norton fought George Foreman for the world heavyweight championship and was knocked out in the second round at Poliedro of Caracas, Venezuela. After an even first round, Foreman staggered Norton with an uppercut a minute into round two, buckling him into the ropes. Norton did not hit the canvas, but continued on wobbly legs, clearly not having recovered, and shortly he went down a further two times in quick succession, with the Referee intervening and stopping the fight. This fight would become known as the "Caracas Caper".

In 1975 at the peak of his boxing career, Norton made his acting debut starring in Dino De Laurentiis blaxploitation film Mandingo, about a pre-Civil War slave purchased to fight other slaves for their masters. After starring in the 1976 sequel Drum Norton went on to bit parts in a dozen other productions.

On September 28, 1976, at Yankee Stadium in New York City, Norton would again fight Ali, who was now the world heavyweight champion since regaining the title with an eighth-round knockout of George Foreman in 1974. Many observers have felt this was the beginning of Ali's decline as a boxer. It was a tough bruising battle for Ali. In one of the most disputed fights in history, the fight was even on the judges' scorecards going into the final round, which Ali won on both the referee's and judges' scorecards to retain the world heavyweight championship. The judges scored the bout 8–7 for Ali, and the Referee scored it 8–6 for Ali. At the end of the last round, the commentator announced he would be "very surprised" if Norton has not won the fight.

Ken Norton was twice voted "Father of the Year" by the Los Angeles Sentinel and the Los Angeles Times in 1977. To quote Norton from his biography, Believe: Journey From Jacksonville: "Of all the titles that I've been privileged to have, the title of 'dad' has always been the best."

In his first defense of the WBC title on June 9, 1978, Norton and new #1 contender Larry Holmes met in a classic fight. After 15 brutal rounds, Holmes was awarded the title via an extremely close split decision. The three judges' cards were as follows: 143–142 for Holmes, 143–142 for Holmes, and 143–142 for Norton. The Associated Press scored it 143–142 for Norton. The March 2001 edition of The Ring magazine listed the final round of the Holmes-Norton bout as the 7th most exciting round in boxing history. As noted above, Holmes-Norton is ranked as the 10th greatest heavyweight fight of all time by Monte D. Cox, a member of the International Boxing Research Organization (IBRO). Holmes went on to become the third-longest reigning world heavyweight champion in the history of boxing, behind Joe Louis and Wladimir Klitschko. Years later, Holmes wrote of his experience that this was his toughest match in over 70 contests.

After losing to Holmes, Norton won his next fight by knockout over sixth-ranked Randy Stephens in 1978 before taking on Earnie Shavers in another compulsory WBC title eliminator fight in Las Vegas on March 23, 1979. It appeared for the first time that Norton's career had perhaps hit a decline, particularly after the Holmes match as Shavers took the former champion out in the first round. But it also created a view that his confidence wasn't good against all-time great hitters Foreman, Shavers and later Cooney. Although Norton himself always denied this and he was past his prime when he was stopped by Shavers and Cooney.

After the fight, Norton decided that at 37 it was time to retire from boxing. However, not satisfied with the way he had gone out, Norton returned to the ring to face the undefeated Randall "Tex" Cobb in Cobb's home state of Texas on November 7, 1980. In an all action back-and-forth fight, Norton escaped with a split decision, with Referee Tony Perez and judge Chuck Hassett voting in his favor and judge Arlen Bynum giving the fight to Cobb.

The win over the title-contending Cobb gave Norton another shot at a potential title-fight, and on May 11, 1981. at Madison Square Garden he stepped into the ring with top contender Gerry Cooney, who, like Cobb, was undefeated entering the fight. Very early in the fight it became clear that Norton was no longer the caliber of fighter he once was, as Cooney's first punch caused Norton's legs to buckle. Norton continued to take shots from Cooney in his corner for nearly a full minute before Perez, who refereed his last fight, stepped in to stop the bout 54 seconds in, as Norton was slumped in his corner. Norton decided to retire following the match and turned his attention to charitable pursuits. Norton's enduring legacy as a fighter is that he is considered second to Joe Frazier as Ali's main nemesis and toughest opponent. Norton fought Ali to three decisions and was never hurt or knocked down. All three bouts were close and subject to controversy. Unfortunately, Norton was less successful against three of the greatest punchers of all time, losing by KO to Foreman and Shavers and by TKO to Cooney. Norton was considered past his prime in boxing from 1979 to 1981.

Norton continued making TV, radio and public speaking appearances until suffering injuries in a near-fatal car accident in 1986. It left him with slow and slurred speech.

Norton is a 1989 inductee of the World Boxing Hall of Fame, a 1992 inductee of the International Boxing Hall of Fame, a 2004 inductee into the United States Marine Corps Sports Hall of Fame, and a 2008 inductee into the WBC Hall of Fame.

The 1998 holiday issue of The Ring ranked Norton #22 among "The 50 Greatest Heavyweights of All Time." Norton received the Boxing Writers Association of America J. Niel trophy for "Fighter of the Year" in 1977.

He appeared along with Ali, Foreman, Frazier and Holmes in a video, Champions Forever, discussing their best times, and in 2000 he published his autobiography, Going the Distance.

In 2001, Norton was inducted by the San Diego Hall of Champions into the Breitbard Hall of Fame honoring San Diego's finest athletes both on and off the playing surface. Norton was also inducted into the Marine Corps Hall of Fame in 2004 and into the California Sports Hall of Fame in 2011.

Norton died at a care facility in Las Vegas on September 18, 2013. He was 70 years old and had suffered a series of strokes in later life. Across the boxing world tributes were paid, with George Foreman calling him "the fairest of them all" and Larry Holmes saying that he "will be incredibly missed in the boxing world and by many".