The victor will replace term-limited Gov. Rick Snyder, a Republican.
Whitmer will now go up against Michigan's attorney general, Bill Schuette, in the November general election.
Whitmer, a former legislative leader, defeated chemical-testing businessman Shri Thanedar and ex-Detroit health director Abdul El-Sayed in Tuesday's primary.
After toppling incumbent Joe Crowley in the NY-14 primary, socialist Alexandria Ocasio-Cortez attempted to use her newfound publicity to secure more socialist victories, stumping for a handful of other Democratic candidates around the country. She was considered the favorite due to her political experience and broad support from organized labor and other key groups.
Abdul El-Sayed from MI, who was on the ticket for governor, fell 20 percentage points behind his Democratic competition, even with both Sanders and Ocasio-Cortez coming out to stump for him.
It was a high-stakes primary for a party that's hoping to ride a "blue wave" back to power in Lansing and is still grappling with internal fault lines exposed during the 2016 presidential election.
The Michigan governorship is a top target for the Democratic Party. "It's time to repeal the retirement tax and yes, to fix the damn roads".
"When it comes to tax cuts for billionaires and unlimited war, we seem to be able to invent that money very easily", she said, "And to me, it belies a lack of moral priority that people have right now, especially the Republican Party".
"Gretchen Whitmer will work every day to make sure Michiganders lives improve, and that's why she's going to be a great governor", said Democratic Governors Association Vice Chair Gina Raimondo.