William Joseph Parnell MacMillan was born in 1881 in Prince Edward Island. He received his early education at Kensington High School and Prince of Wales College before entering McGill University in 1904. At his graduation in 1908, he was recipient of the Holmes Gold medal, a prize given to the best all round student.

Upon his graduation, MacMillan returned to P.E.I. where he set up a medical practice in Kinkora and married Mary. C. McDonald. He soon moved his practice to Charlottetown and eventually specialized in radiology, managing the Radiology department at the Charlottetown Hospital.

Dr. MacMillan led a very active public life. He was elected to the P.E.I. legislature in 1921 and was the Provincial Health officer for P.E.I. from 1911 to 1930. He was P.E.I.'s first Minister of Public Health and first Minister of Education. He was elected Premier of P.E.I. in 1933, serving a two-year term. From 1935 to 1950 he was leader of the opposition.

Other positions held by Dr. MacMillan included: Honorary-Counselor of the Canadian Red Cross, Governor of St. Dunstan's University, President of the Children's Aid Society, member of the Rotary Club, and Chief of staff at the Charlottetown Hospital. He received the Senior Member emblem from the Canadian Medical Association in 1952. In 1957, he was appointed Lieutenant-Governor of P.E.I. but died shortly before taking office.