 Cosbie attended Saint Francis High School in Mountain View, California, where he played football, basketball, baseball and ran in the two-mile relay in track.In 1973, he helped the Lancers reach the finals of the Central Coast Section playoffs, before losing the title game 32-30 to Saratoga High School.In 1974, he was part of the first Santa Clara Valley All-Star Football Game, playing with the North team.