The service, called Maven, debuted Thursday in Ann Arbor, Mich., and will initially focus on serving students and faculty at the University of Michigan. GM vehicles are to be available at 21 spots throughout the city.
Users can rent a car through a free smartphone app, which is also used to unlock the vehicle, GM said.
The company said Maven would expand to additional cities later this year.
"With the launch of our car-sharing service through Maven, the strategic alliance with ride-sharing company Lyft and building on our decades of leadership in vehicle connectivity through OnStar, we are uniquely positioned to provide the high level of personalized mobility services our customers expect," GM President Dan Ammann said in a statement.
GM and other major car manufacturers have been exploring new ways to move toward an automotive future in which substantial numbers of drivers will not own cars.
In June, Ford launched a car-sharing program that offers buyers a new way to offset the pains of ownership by tapping into what is essentially an Airbnb on wheels — letting strangers rent their cars for short periods of time. In Germany, GM previously had launched a Car-Unity app that lets owners of any brand rent their vehicles to Facebook friends or to people in the app's network.
Other companies are testing the waters.
"Society and the automotive industry are undergoing radical change," said Peter Schwarzenbauer, who heads BMW's Mini and Rolls-Royce brands. "So it makes sense for us to offer a car-sharing option for Mini starting in 2016."
Bob Carter, who heads Toyota's U.S. sales, says his company is gauging how car- and ride-sharing companies such as Uber will affect their business. In San Francisco, for example, "we will see business models develop around access to cars but not ownership," he said.
Analysts cautioned that although GM's move is a bold one, it's only another iteration in the transforming transportation industry.
"The Maven launch will give GM broader reach in this dynamic atmosphere, but nobody knows which of these alternative transportation systems holds the greatest long-term potential," Kelley Blue Book senior analyst Karl Brauer said. "This is why we're seeing a shotgun approach coming from multiple automakers."
The launch of Maven comes days after the news that GM acquired the assets of ride-hailing company Sidecar Technologies Inc. The San Francisco company had shut down at the end of last year.
Also this month, GM invested $500 million in the ride-sharing service Lyft. The automaker will also take a seat on Lyft's board, and the two companies plan to collaborate on an Autonomous On-Demand Network that would allow users to reserve a self-driving car, much as they do with Lyft.
Separately on Thursday, GM announced its financial results for 2015. The company reported global sales of 9.8 million vehicles in 2015, representing a third straight year of growth for the Michigan-based automaker. New vehicle deliveries in North America rose 6% from a year earlier. In China, they rose 5%.