The holdup happened around 12:00 p.m. at the facility located at 5201 Parkside Avenue in Philadelphia's Fairmount Park.
In a statement from the Mann Center, president Catherine Cahill said the men stole the money from the offices of Wolfgang Puck Catering, which operates on the premises.
Police say the men tied scarves to cover their faces and were armed with a silver revolver. Police say the men laid the accountant on the floor and tied up his arms and legs.
That accountant was not hurt.
"They just asked if there was any more money here. They said 'Keep your mouth shut' as he was on the ground. But no assault whatsoever," said Lt. John Walker of the Philadelphia Police.
The suspects fled in a vehicle, possibly a light blue minivan, toward Belmont Avenue.
There are two entrances to the grounds covered by security, but police believe the armed robbers climbed the back fence to get in and out.
The only description being released so far - two black men around 6 feet tall and weighing 170 pounds. They were in and out in less than five minutes and seemed to know the operation. Authorities here are examining the angle of an inside job.
"We'll be taking a close look at operations here," said Lt. Walker.
Cahill said the "Death Cab for Cutie" concert scheduled for Friday night would go on as planned.