The incident happened Friday morning around 7:30 at the Max Federal Credit Union in the 4700 block of Mobile Highway.
The 43-year-old victim told authorities a man approached her with a gun and robbed her of her money.
A quick-thinking witness saw the robbery and wrote down the robber's vehicle description and tag number.
Chief Art Baylor said, "It is because of the actions and involvement of the good citizens in the community that we are able to locate and arrest violent offenders like this. I hope all citizens take this witness' actions as a lesson and get involved to help the Police Department catch these criminals. As we have said before, citizens working together with the police is what it takes to make a difference."
Detectives identified the suspect as Clavon Williams, 20, of Northington Drive. Williams was taken into custody without incident.
He's now charged with Robbery 1st Degree and is being held at the Montgomery County Detention Facility under $50,000 bond.