Brown says members of the Gray campaign paid him cash and promised him a job for his support during the race.
Gray says he was surprised, shocked and appalled by the allegations, which were splashed across the front page of Sunday's Washington Post.
At an impromptu press conference held late Sunday, Gray denied Brown's allegations, saying while he did promise the candidate a job interview, he never promised a job.
Brown later landed one with the city's Medicaid agency and was later fired for undisclosed reasons.
The mayor also rejected the claim that Brown, who was an unrelenting critic of then Mayor Adrian Fenty on the campaign trail, received any cash from the Gray campaign.
"I can't even imagine engaging in such reprehensible behavior and I know absolutely nothing about anyone in my campaign who did that," Gray says.
The Post article details dozens of phone calls between the Gray campaign and Brown, but it could not substantiate the cash payment allegations.
Speaking about this controversy and some of the other scandals that have enveloped city hall in recent weeks, Gray acknowledged his administration has made some missteps and says he wants to move on with the business of the city.