| Order: | 18th |
| Office: | Premier of Prince Edward Island |
| Predecessor: | James D. Stewart |
| Successor: | Walter Lea |
| Lieutenant Governor: | Charles Dalton George DesBrisay DeBlois |
| Term Start: | October 14, 1933 |
| Term End: | August 15, 1935 |
| Office1: | Leader of the Conservative/Progressive Conservative of Prince Edward Island |
| Predecessor1: | James D. Stewart |
| Successor1: | Reginald Bell |
| Term Start1: | October 14, 1933 |
| Term End1: | June 26, 1950 |
| Office2: | MLA (Councillor) for 5th Queens |
| Predecessor2: | Gavan Duffy |
| Successor2: | C. St. Clair Trainor |
| Term Start2: | July 26, 1923 |
| Term End2: | July 23, 1935 |
| Predecessor3: | C. St. Clair Trainor |
| Successor3: | Alex MacIsaac |
| Term Start3: | May 18, 1939 |
| Term End3: | May 25, 1955 |
| Birth Name: | William Joseph Parnell MacMillan |
| Birth Date: | 24 March 1881 |
| Birth Place: | Clermont, Prince Edward Island |
| Death Place: | Charlottetown, Prince Edward Island |
| Nationality: | Canadian |
| Spouse: | |
| Party: | Conservative/Progressive Conservative |
| Children: | 6 |
| Alma Mater: | Prince of Wales College McGill University |
| Occupation: | physician and surgeon |
| Profession: | Politician |
| Cabinet: | Minister without Portfolio (1923–1927) Minister of Education (1931–1933) Minister of Public Health (1931–1935) Provincial Secretary-Treasurer (1933–1935) |
William Joseph Parnell MacMillan (March 24, 1881 - December 7, 1957) was a physician and Prince Edward Island politician.
Born in Clermont, MacMillan was a graduate of Prince of Wales College and the McGill University Faculty of Medicine.
After running a medical practice for several years, MacMillan entered politics in 1923 winning a seat in the provincial legislative assembly as a Conservative. In 1932 he became the province's first minister of health and education. As minister he rebuilt Prince of Wales College and the Provincial Mental Hospital which had both been destroyed by fire. He also secured a Carnegie Foundation endowment that enabled the government to establish a provincial library system.
When Premier James D. Stewart became ill, MacMillan served as acting premier and then became the 18th premier of Prince Edward Island when Stewart died in 1933. MacMillan's government implemented relief programs and increasing government spending to help dampen the impact of the Great Depression.[1] Nevertheless, his government was swept from power in the 1935 election which saw the Liberals capture every seat in the legislature. He returned to the legislature in 1939 and remained leader of the Conservative Party until the late 1940s and a member of the legislature until he lost his seat in the 1955 election.
MacMillan was named the province's lieutenant governor on November 22, 1957 but died before being sworn in.