MacMillan, Jr. was the son of Cargill MacMillan, past president of Cargill, and Pauline Whitney MacMillan. His career with Cargill spanned 38 years, starting as a trainee in 1950. He had positions in Buffalo and New York City, New York, U.S.; Chicago, Illinois, U.S., and Minneapolis, Minnesota, U.S.
He is credited with building the Cargill Office Center in Minnetonka, Minnesota, U.S. He served on the Cargill board of directors from 1963 to 1996 and chaired the finance committee.
MacMillan, Jr. was born in Minneapolis and grew up in Wayzata, Minnesota, U.S., He attended the Blake School in Minneapolis, and served in the U.S. Air Force. He received bachelor’s degree in economics from Yale University.
He was involved in many organizations, serving on the boards of Abbott-Northwestern Hospital, Minneapolis Society of Fine Arts, Twin Cities Public Television, Minneapolis Institute of Arts, Macalester College, Greater Minneapolis Chamber of Commerce, United Way of the Minneapolis Area, the Johnson Institute, Northwestern National Bank of Minneapolis, Community First Bank, Minnesota Outward Bound School, and the YMCA.
MacMillan, Jr. is survived by his wife, Donna, and their six children, 24 grandchildren, and two great-grandchildren. He is survived by his brother, Whitney, and sister, Pauline, and former wife, Marnie. His sister, Alice, preceded him in death.
There will be a service celebrating MacMillan, Jr.'s life on Dec. 3 at 3 p.m. at Wayzata Community Church, 125 Wayzata Boulevard E., Wayzata, Minnesota, U.S. Reception to immediately follow at Woodhill Country Club. In lieu of flowers, the family requests that memorials be directed to the Minneapolis Institute of Arts, 2400 Third Avenue South, Minneapolis, Minnesota 55404, or to a charity of your choice.