Nicholas Ulrich Mayall (May 9, 1906 – January 5, 1993) was an American observational astronomer. After obtaining his doctorate from the University of California, Berkeley, Mayall worked at the Lick Observatory, where he remained from 1934 to 1960, except for a brief period at MIT's Radiation Laboratory during World War II.

During his time at Lick, Mayall contributed to astronomical knowledge of nebulae, supernovae, spiral galaxy internal motions, the redshifts of galaxies, and the origin, age, and size of the Universe. He played a significant role in the planning and construction of Lick's 120-inch (3.0 m) reflector, which represented a major improvement over its earlier 36-inch (0.91 m) telescope.

From 1960, Mayall spent 11 years as director of the Kitt Peak National Observatory until his retirement in 1971. Under his leadership KPNO, and the Cerro Tololo Inter-American Observatory, developed into two of the world's top research observatories, equipped with premier telescopes. Mayall was responsible for the construction of the 4-meter (160 in) Kitt Peak reflector, which was named after him. When Mayall died in 1993, his ashes were spread high on an empty ridge of Kitt Peak.

Mayall's father, Edwin L. Mayall Sr., was an engineer for a manufacturing company in Illinois. His mother, Olive Ulrich Mayall, despite never attending college herself, set high educational standards for Mayall and his younger brother (Edwin Jr., born 1907). Sometime between his brother's birth and 1913, the family moved to California's Modesto area, where Mayall entered first grade. Some time before 1917, they moved again, to Stockton where they remained until 1924 and Mayall's graduation from Stockton High School (except for a brief return to Peoria, Illinois during 1918–1919). During this period, presumably during his high school years, Mayall's parents divorced.

During his senior year, in the fall of 1923, Mayall was secretary of the school science club and set up a club visit to the Lick Observatory. His father permitted him to use his car, a Moline Knight, to transport the club members up the dirt and gravel winding mountain road leading to the observatory. This was Mayall's first visit to the observatory where he would spend much of his career. After visiting, he read all the astronomy books available in the local libraries, although he did not at that time imagine making astronomy his profession.

Mayall began college in the fall of 1924 at the University of California, Berkeley, studying for a degree in mining. He took up residence with his mother in an apartment on Durant Avenue, and worked at the UC Berkeley library to help support them both. Mayall generally did well at university, and was eventually elected to the Sigma Xi and Phi Beta Kappa honor societies. However, at mid-term examinations of his second year, he achieved poor grades in mineralogy and chemistry laboratory. At a meeting with the dean to discuss his grades the latter became aware that Mayall was color blind, which prevented him from observing small color changes in bead and flame tests, and also kept him from seeing small color changes in precipitations and titrations. Mayall's adviser recommended that he change his major, as he would not be able to graduate as a mining engineer with such a handicap.

Mayall's mother encouraged him to study whatever interested him the most, and to do it well, so he considered astronomy as an alternative to mining. After asking many professors in the astronomy department whether they enjoyed their work and whether they made a satisfactory wage, and being content with their answers, he transferred to the College of Letters and Science to major in astronomy. This did not set him back in his degree requirements because almost all of his first year studies had been in basic physical sciences and mathematics. Eventually Mayall discovered that he greatly enjoyed astronomy, and decided upon a course of graduate level study followed by a career as a research scientist.

After graduating in 1928, Mayall decided to remain at Berkeley, as it had the best astronomy graduate program of the day. However, he took a hiatus from pursuing his advanced degree and went to work as a human computer at the Mount Wilson Observatory from 1929 to 1931, where he assisted prominent astronomers including Edwin Hubble, Paul W. Merrill, and Milton L. Humason. This activity resulted in him co-authoring papers on Pluto's mass and orbit with Seth Barnes Nicholson and others, shortly after Pluto's discovery