Leroy Still Jr., 66, of the 300 block of East Hector Street in Conshohocken, was arrested on Thursday.
Another man, 48-year-old John Jones, aka Christopher Jones, remains at large. Police went to his home in the 2900 block of North Taylor St. in Philadelphia on Thursday to arrest him, but he was not here.
The robbery happened on the morning of Thursday, October 10th in the 1300 block of East Ridge Pike.
Police say the 84-year-old owner of a real estate business 'buzzed' two men into the building, now identified as Still and Jones.
The men allegedly asked about renting a home, but were told there was nothing available.
Then, police say, one of the men pulled out a gun while the other tied the victim to his desk chair.
The man with the gun then reached into the victim's pockets and took his money.
They then ripped the phone off his desk and threw it across the room before leaving.
The 84-year-old victim was not injured.
Anyone with information on Jones' whereabouts is asked to contact the Plymouth Township Police Department at 610-279-1901.