Jemile Weeks singled in the tying run for Oakland in the eighth and the Athletics then won their fifth in a row by getting to relievers Oliver Perez and Steve Delebar in the 11th.
Perez (0-1) gave up a single to Josh Reddick leading off the 11th and, after Yoenis Cespedes flew out, Seth Smith reached on an infield single.
Delebar came on to replace Perez and Carter his his second pitch to left-center for his third homer of the season. Seattle suffered its third straight loss.
The Carter homer brought Jordan Norberto the first win of his major league career. Norberto (1-1), who appeared in his 65th game scattered over three seasons, struck out the side in the top of the 11th.