Posey, a native of Leesburg, Georgia, returned after missing two games with discomfort in his left knee. He lifted a first-inning fastball from Max Fried high toward the right-field foul pole. Fried watched with an incredulous look as the fly ball remained fair and fell into the first row of the right-field seats.
Posey's homer drove in Austin Slater, who was hit by a pitch to lead off the game.
Giants manager Gabe Kapler had said before the game Friday that catcher Curt Casali, the fill-in starter for Posey who left Thursday night's 3-2 win at New York because of dizziness, was available off the bench. Kapler said tests on Casali "didn't find any evidence of a concussion."
Posey, the 2012 National League MVP, was under no restrictions, Kapler said. He had not played since being removed from Tuesday's game.
"We've taken every precaution to get him ready to play nine innings plus," Kapler said.
Kapler said Friday he had permission from Solano to disclose that Solano was fully vaccinated when he tested positive. Kapler said he spoke with Solano on Friday and said the second baseman was disappointed to be away from the team.
The plan is to "keep him active so when he comes out of the quarantine he's not deconditioned," Kapler said.
Hitting coach Justin Viele was away from the team because of contact tracing, Kapler said, adding that Viele has tested negative.
The Associated Press contributed to this report.