Standing on the convention stage with an empty chair, Eastwood carried on a sometimes rambling conversation with an imaginary President Barack Obama.
The actor and director talked about Oprah Winfrey, Obama's unfulfilled promise to close the U.S. prison at Guantanamo, the wars in Iraq and Afghanistan and lawyers.
At one point, Eastwood talked about letting Obama go and making a change.
"When somebody doesn't do the job, you gotta let 'em go," Eastwood said. The tough-guy actor of "Dirty Harry" fame drew a finger across his throat.
The crowd cheered Eastwood's entrance and shouted his famed catchphrase, "Go ahead, make my day." But backstage, stern-faced Romney aides winced at times as Eastwood's remarks stretched on.
On a night where virtually every moment was scripted, Eastwood was among the only speakers not reading from a teleprompter as he spoke.
The Oscar-winning director of "Unforgiven" and "Million Dollar Baby," Eastwood criticized Obama for failing to turn the economy around and for wanting to close the Guantanamo Bay prison for terror suspects.
Eastwood says Obama has failed to deliver on his promises and it's time for Romney and his running mate, Paul Ryan, to take over.
He entertained the audience with a mock interview with Obama, posing questions and pretending that Obama had told Eastwood to shut up.
Eastwood endorsed Romney earlier this month at a campaign event in Sun Valley, Idaho.
Associated Press writers Kasie Hunt and Steve Peoples contributed to this report.