The alleged carjacker grabbed the woman from behind while she attempted to enter her vehicle behind a condo building at West 15th Street and South State Street at approximately 11:53 a.m., police said.
The woman sustained head injuries and could not recall details of the incident, police said. Her 2008 BMW was missing from the scene.
The building's doorman, Tony Thomas, and several residents rushed to her aid.
"When I got out there, the young lady was laying there," Thomas said. "In pain, crying out for help."
The woman is an employee of a nearby Firestone Complete Auto Care and was attacked during her break.
"She said she was walking to her car," Thomas said. "There was a guy standing there and she tried to enter her car, knocked her down, took her keys and pulled off. But apparently he hit her and pulled her a little bit, her leg was broken."
The woman was transported to Northwestern Memorial Hospital in stable condition, authorities said.
Denise Goldfarb parked her car in the same alley this weekend while she was house-sitting for a friend.
"She said to make sure to keep an eye on the car and obviously ourselves as well," Goldfarb said. "It's very disconcerting and it's very scary because it's happening way, way too much."
No one was in custody Saturday.
Saturday night, a 31-year-old woman was robbed and carjacked at gunpoint in the Lakeview neighborhood on the North Side.
At 7:28 p.m., the 31-year-old was sitting in her parked 2016 Chevrolet Cruze in the 1000-block of West Barry when a male walked up and ordered her to open the driver's side door, implying he had a handgun, according to Chicago police. A second male then walked up to the victim and stole her backpack, which had her laptop, cellphone and cash inside.
The suspects drove off in the Chevrolet, which was later found unoccupied in the 3000-block of West Lexington, police said.
Sun-Times Media Wire contributed to this report.