Mr Howard told the Manhattan courtroom under cross-examination on Wednesday: "If he is going to carry an illegal firearm, there is no reason for me to be there.
Prosecution witness Leonard Curtis Howard, a former prison officer, said he would never have associated himself with Mr Combs if the defendant had carried a firearm.
Mr Combs is charged with possession of a weapon in connection with a 1999 shooting at a New York nightclub, in which three people were injured.
He is also charged with trying to bribe his driver to take the blame and could face up to 15 years in jail if found guilty.
Mr Howard told the Manhattan courtroom under cross-examination on Wednesday: "If he is going to carry an illegal firearm, there is no reason for me to be there.
"I'm not going to put my job in jeopardy for Mr Combs."
The shooting allegedly happened after a man insulted Mr Combs and threw money in his face.
Defence lawyer Benjamin Brafman had earlier moved unsuccessfully for a mistrial, because assistant district attorney Matthew Bogdanos used Mr Howard's grand jury testimony to contradict his trial testimony.
At the trial, Mr Howard testified he had seen Mr Combs frisked on occasion when entering nightclubs.
But he told the grand jury he had never seen the rap star searched in that situation.
This is an important distinction, as prosecutors allege the Grammy-winning rapper carried a gun into the nightclub and fired a shot on the night of his arrest.
Mr Combs is not accused of shooting anyone, but he was arrested after he fled the club with his girlfriend, actress and singer Jennifer Lopez, who has not been charged.
Mr Bogdanos, prosecuting, said: "When a witness lies as egregiously as Mr. Howard did, it is incumbent upon the (questioning) attorney to point that out to the jury."
Co-defendant Jamal Barrow, 21, faces charges of attempted murder and gun possession, while bodyguard Anthony Jones, 34, is accused of having the loaded gun by his feet.
Detective William Wallace, a crime scene specialist, told the court he found a bullet fragment from a gun when searching the club after the shooting.
The court was told on Tuesday at the trial's opening that Mr Combs "fired a shot" into the ceiling of Manhattan's Club New York.
Mr Bogdanos said: "Witnesses will say they saw the muzzle flash."
Mr Combs' defence team said the prosecution was picking on their client because of his fame.
Mr Brafman is best known for defending leading mafia figures. The defence team also includes Johnnie Cochran, who represented OJ Simpson.
Court officials say they expect the trial to run for between five and six weeks.
The three people injured in the shooting are each seeking a $100m (£67.8m) settlement.