The private, unoccupied ambulance was in the 100-block of West Oak Street before noon when the man reportedly entered it and drove east, passing Michigan Avenue.
He then exited the ambulance in the 200-block of East Lake Shore Drive in Streeterville and took a 47-year-old man's vehicle, police said.
The suspected thief emptied items from the ambulance into the other stolen vehicle and fled the scene, according to police.
Area Three Detectives are investigating. It was not immediately clear later Saturday if anyone was in custody.