Preckwinkle, a longtime Chicago alderwoman, beat out Republican Roger Keats and Green Party candidate Tom Tresser.
Preckwinkle was in the lead from the start. With 76-percent of the votes in Tuesday night, Preckwinkle took the lead with 69-percent of the votes. Keats had 27-percent and Tresser had 4-percent.
She was endorsed by two major newspapers and won the primary without the official endorsement of Democrats.
"With all of your help I'm confident we can meet the challenges in front of us," said Preckwinkle in her victory speech. She also said she will "clean up county government" and end patronage and "fraud that has cost taxpayers millions of dollars."
Preckwinkle will take over from Todd Stroger, who followed his father into the post and faces allegations of corruption and patronage.
With a $3 billion budget, Cook County is the nation's second largest in the country.