William Duncan MacMillan (July 24, 1871 – November 14, 1948) was an American mathematician and astronomer on the faculty of the University of Chicago.

He published research on the applications of classical mechanics to astronomy, and is noted for pioneering speculations on physical cosmology.

[3] For the latter, Helge Kragh noted, "the cosmological model proposed by MacMillan was designed to lend support to a cosmic optimism, which he felt was threatened by the world view of modern physics.

He performed his graduate work at the University of Chicago, earning a master's degree in 1906 and a PhD in astronomy in 1908.

This was uncontroversial at the time, and indeed in 1918, Albert Einstein had also sought to adapt his relativity theories to the model using a cosmological constant.

In 1948, a new version of a steady-state cosmology was proposed by Bondi, Gold, and Hoyle that was consistent with the measurements on distant galaxies.

[3][4] With the observation of the cosmic microwave background (CMB) in 1965, steady-state models of the universe have been rejected by most astronomers and physicists.

[8] In an Associated Press report, MacMillan speculated on the nature of interstellar civilizations, believing that they would be vastly more advanced than our own.