Winfrey called Abrams a "change-maker" who represents the values of all Georgians.
The audience cheered as Winfrey said: "I'm here today because of the men and because of the women who were lynched, who were humiliated, who were discriminated against, who were suppressed, who were repressed and oppressed".
The governor's race has been plagued by accusations of voter suppression.
Abrams is a Yale-educated attorney who served as minority leader in the state legislature.
Both candidates have taken to describing the race as a battle for "the soul of our state". "I came for myself and I approve this message".
Ms Winfrey, who has dismissed rumours she is planning to run for President in 2020, also endorsed Democrat Barack Obama before the 2008 election. I'm not trying to test any waters - don't want to go in those waters.
"I don't want to go in those waters!" "She cares about medicaid expansion, keeping families together", she said.
"I think Brian's going to be a great governor of Georgia", Trump said.
"Surprised?" the woman, named Denise, said.
Vice President Mike Pence was forced to remind Republican voters in Georgia on Thursday that he's important. President Trump will appear with Kemp on Sunday.
Georgia, however, is home to a booming film industry, thanks to more than $1 billion in tax credits over the past decade, according to the Atlanta Journal-Constitution.
Winfrey did not mention Pence or the frequent GOP broadsides about "Hollywood" outsiders.
The focus of her speech, however, wasn't Abrams.
When a woman opens the door, she lets out an excited cry. There's something about her that's just magnetic.
The pitches from Pence and Winfrey clearly fit Abrams' and Kemp's respective strategies.
Trump's scheduled appearance sidelined the second and final scheduled between Kemp and Abrams.
Winfrey and Abrams greet a crowd gathered for a town hall conversation in Marietta, Ga.