Ray Mancini strolled through the entrance of a Manhattan steakhouse the other day and eased into a cozy booth near the front. The lunch crowd hardly noticed.

Some 30 years ago, anonymity simply would not have been possible for the prizefighter known as “Boom Boom.” Mancini was at the top of his game during an era when boxing captivated American audiences like few other sports, and fans couldn’t get enough of the lightweight champion of the world.

On his best days, Mancini required mere minutes to finish opponents, like the time he scored a first-round TKO of Arturo Frias in 1982 to claim the World Boxing Association belt. Or when he dispatched Bobby Chacon in the third round two years later in his last of four successful title defenses.

“My father used to say things to me all the time like, ‘Always remember, one day the headline, next day the breadline,’ ” Mancini said. “Same people calling you champ on the way in are calling you bum on the way out. Don’t ever take this serious, meaning the fame. I didn’t. I had fun with it.”

Mancini spoke tenderly and at length about his father, Lenny, from whom he took the nickname “Boom Boom.” He beamed when discussing his own ruthless style in the ring — “always moving forward,” he likes to say — in which 23 of 29 victories came via knockout.

And just as Mancini never ducked an opponent, he freely detailed some of his bleakest moments, including divorce from his first wife, the shooting death of his brother and most notably his bout against Duk-koo Kim on Nov. 13, 1982, in Las Vegas that resulted in tragedy.

In a match that ranks among the most brutal and compelling of all-time, Mancini administered such punishment en route to a 14th-round knockout that Kim, who fought valiantly in his own right, died in a nearby hospital four days later. Kim fell into a coma shortly after the fight, and emergency surgery for a subdural hematoma was not successful.

Mancini, 52, is asked about the fight with Kim more than anything else in his Hall of Fame career.

“I made peace with it long ago,” Mancini said. “My faith carried me through it.”

Mancini’s life is the subject of the documentary entitled “The Good Son,” based on the book of the same name by Mark Kriegel. The film is scheduled for theatrical release Aug. 9 and is available currently on video on demand and iTunes.

The producers of “The Good Son” are longtime friends Chris Tavlarides and Jimmy Lynn, both of whom have strong ties to Washington.

“The reality is without the genuineness of Ray’s personality and his persona, this would have been a very difficult project to do,” said Tavlarides, a George Washington University graduate. “So it was extremely rewarding for me, and I know it was extremely rewarding for Jimmy. Ray has a gift that very few famous people have, and that is he doesn’t quite realize how famous he really is.”

Mancini said he’s that way because of his father, whose boxing career was never the same after World War II, after he was wounded in action from an artillery shell; it left him with a broken shoulder and fractured left arm. Shrapnel ripped through his left arm, left leg and right foot.

Lenny Mancini admired gracious fighters like Joe Louis and Sugar Ray Robinson. He raised his son to comport himself accordingly.

Muhammad Ali “was the first to use that psychological warfare,” said Ray Mancini, who finished his career with a 29-5 record. “My father had no hatred for him, but he’d be yelling at the TV to stop all that talking. I think that’s what missing now. The gentlemanly thing is long gone.”

Mancini allowed cameras to capture that authentic side during the movie's most poignant scene when he meets Kim’s son, Jiwan, and fiancee, Young Mee Lee, for the first time. Lee was two months pregnant with Jiwan at the time of Kim’s death.

Mancini is waiting at his home in Santa Monica, Calif., on a summer afternoon in 2011. A white SUV pulls up in front carrying Jiwan Kim and his mother, and Mancini greets them outside. He introduces his children to them, and all proceed to have dinner together in the back yard, sipping on wine from Mancini’s Southpaw winery.

Mancini confesses he harbored guilt for Jiwan never getting to meet his father and that his passion for boxing waned when he learned of Kim’s passing. Then Jiwan replies:

“Now I can tell you that when I saw the fight the first time I felt some hatred to you. I think it was not your fault. You deserve. . . . Maybe now your family will be more happy.”

Mancini was in Cleveland recently for a screening of “The Good Son,” and he recalled one particularly curious fan asking him how many takes were required to capture that moment.

“I said, ‘No, my friend.’ I said I told Jesse James Miller, my director, ‘One shot to get this,’ ” Mancini said. “I can’t do this again.”

Always moving forward. That’s Ray Mancini.