Washington Park is about 10 miles east of St. Louis.
Police believe someone flagged Thornton down and asked for a dollar. Residents said he was known for driving through the city and stopping to help people in need.
"He was just a people person, you know. Wherever... uh? you know...like I say, if you come up and you needed something, I mean, he was there. He'd give it to you right there with no hesitation," said John Thornton, Jr., mayor's son.
Police said a witness heard two gunshots before Thornton's car struck the tree. The gunman then limped to a waiting car and fled.
Police have taken two people into custody in connection with the shooting.