 Wirth was originally drafted by the San Francisco Giants in the 3rd round of the 1974 Major League Baseball Draft.After four seasons in their organization, Wirth was part of one of the most famous trades of 1978, when the Giants traded seven players—Wirth, Gary Alexander, Gary Thomasson, Dave Heaverlo, John Henry Johnson, Phil Huffman, Mario Guerrero—and $300,000 to the A's for pitcher Vida Blue.Wirth started the 1978 season with Oakland, but was sent to the minors in June.