James McIntyre, known professionally as Jimmy Mack, was a high-school friend of John Linnell and John Flansburgh. He is credited with exposing the Johns to music that diverged from the mainstream, such as the Ramones. McIntyre also worked alongside Linnell and Flans on the Lincoln-Sudbury High School newspaper, The Promethean. Due to his influence on the Johns' sensibilities, and their close friendship, McIntyre is often considered to be the "third Giant".
Although McIntyre dropped out of high school before graduating, he had a successful career in the music industry. He worked under the pseudonym Jimmy Mack, and kept in touch with the Johns throughout the rest of his life. McIntyre was openly gay and passed away at a young age due to HIV/AIDS. Consequentially, Flansburgh and Linnell have often participated in raising funds for AIDS research. TMBG's debut album is dedicated to him.