King, who is black, said the word as he argued that African-Americans cannot achieve success by emulating white people, as they will remain "negroes."
"If you're poor, you are a poor negro -- I would use the n-word -- but if you're rich, you are a rich negro. If you are intelligent, intellectual, you are intellectual negro. If you are a dancing and sliding and gliding n***** -- I mean negro -- you are a dancing and sliding and gliding negro," King said, laughing along with the crowd after the slip-up. "You're going to be a negro 'til you die."
Trump, who was already smirking as he listened to King, didn't change the expression on his face, but turned slightly uncomfortably to his special counsel Michael Cohen and the church's pastor, Darrell Scott, who head up Trump's diversity coalition.
Trump, who said he personally invited King to introduce him, was sitting in a chair onstage just a few feet away from King when he uttered the word.
Taking the stage moments later, Trump called King a "phenomenal persona" and said: "Ah, there's only one Don King."
During his speech, King praised Trump as "fearless," "courageous and brave" and said he believed Trump would "take this system apart" and "create a whole new system."
"The system is corrupt, the system is rigged, the system is sexist, the system is racist," King said, arguing that Trump would bring the country "back to inclusiveness."
King's mere presence at Trump's side is controversial: King was convicted of second-degree murder for stomping a man to death in 1966. The governor of Ohio later pardoned King for the crime in 1983.
Wednesday's moment came as Trump pitched himself as a president who would help bring jobs and safety to impoverished minority communities during the event at Scott's church. There, Trump was flanked by a number of his prominent black surrogates, including Ben Carson and former "Apprentice" star Omarosa Manigault.
Later, as Trump fielded questions from Scott on a range of issues affecting religious and minority communities, King stood on stage by Trump, interjecting infrequently to shout about the "system."
Warily eying King, Manigault suggested King take a seat, but the boxing promoter declined.
Moments later, as Trump's supporters gathered around him to pray for Trump, King grabbed Trump's wrist and raised their arms above their heads, as he would with a champion boxer.
Trump appeared to let his hand go limp, and then, unsure what to do, closed his hand to make a fist.
Just a few hours later, King flew with the real estate mogul aboard Trump's plane to his rally in Toledo, Ohio, where he introduced GOP vice presidential nominee Mike Pence.