Police say 30-year-old Rahmeek Younger hid the weapon in a bag and brought it into the office in Downtown Brooklyn just before 2 p.m. Monday.
Another client spotted the weapon in the bag and told an HRA peace officer, who called 911.
Officers responded, and the suspect allegedly ran.
Police caught up with him, grabbed him and Tasered him.
Authorities say the gun and the clown mask were found in his bag.