Mackillop was born in Northampton, MA and received his BA and MFA degrees from Tufts University and the Boston Museum of Fine Arts School. An artist and teacher, MacKillop taught at the Roxbury Latin School in Boston (1965-1973) and UNC Charlotte from 1973 to 2003. His works were figurative and incorporated social commentary that explored the male form in terms of autobiographical images or archetypal symbols. He exhibited throughout New England and the Southeast for over forty years. Notable solo and group exhibitions include shows at the Weatherspoon Art Museum, Alternative Museum in New York City, Decordova Museum of North Carolina, and the Mint Museum of Art. In 2017 his family established three endowments at UNC Charlotte in his memory, supporting student scholarships and a bi-annual exhibition of alumni work.