Milton Humason was born on August 19, 1891, at Dodge Center, Minnesota. His entry into the field of astronomy was a testament to persistence. Sent to summer camp at Mount Wilson Observatory when he was fourteen, Humason became so fascinated by the activities there that he somehow convinced his parents to let him drop out of school to return. He ended up working as a mule driver in the area for a few years, before he left to work for a relative in 1911. When he learned of an opening for a janitor at Mt. Wilson Observatory in 1917, he took the job and began working his way up the ladder.

Astronomers Edwin Powell Hubble and George Hale (1868-1938) had noticed Humason's devotion to astronomy and encouraged him. He progressed rapidly and was named to the scientific staff in 1919. His first project was an unsuccessful yet noteworthy search of the sky for planets beyond the orbit of Neptune. He would have discovered Pluto, which he had actually photographed, had its image not fallen on a flawed part of the photographic plate.

Humason's most important work was done jointly with Hubble in 1928, when he systematically began to observe the speed at which the distant galaxies are receding from us. This is done by observing the red shift of light from a galaxy; the farther the light is shifted to the red end of the spectrum, the faster the galaxy is receding. He took measurements for many hundreds of galaxies, developing new techniques along the way.

Working with others in 1956, he used his data to refine Hubble's law. Hubble's law states that the speed at which a galaxy is receding from us is directly proportional to its distance from us. The new refinement allowed for greater galactic velocities in the distant past, which corresponded closely to the theory put forward by Georges Henri Lemaître and George Gamow, who had proposed that the universe was created with a big bang.

Humason died on June 18, 1972, in Mendocino, California.