Illinois State Police said in a statement Aaron Jackson of Washington Park was arrested in the Relax Inn motel in Alorton at 3:55 p.m. by Alorton police. Jackson was picked up on an outstanding warrant after police received a tip from the public, state police said.
Jackson was being held on $1 million bond.
Thornton, 52, was found in his car, which had left a city street and crashed into a tree. Police said he had been shot twice in the chest at close range.
At the time, investigators said the mayor had been shot by someone who wanted to rob him.