Leuschner was born in the US but raised in Germany. He returned to the US for university studies, graduating from the University of Michigan in 1888 with a degree in mathematics. He then became the first graduate student at Lick Observatory, but due to conflicts with his advisor, Lick director Edward S. Holden, he left Lick before finishing his Ph.D. Leuschner subsequently returned to Germany and attended the University of Berlin, where in 1897 he earned his doctorate with a highly praised thesis on the orbits of comets.
He returned to California as an associate professor in astronomy at UC Berkeley, where he remained for over half a century. He founded an observatory there for student instruction, later renamed in his honor Leuschner Observatory. Together with Lick director James E. Keeler, Leuschner shaped the combined graduate program at Berkeley and Lick into one of the nation's foremost centers of astronomical education. Leuschner's own research continued to focus on the orbits of asteroids and comets; this subject required tremendous amounts of detailed computation, which made the work well-suited to be shared with a long series of students, many of whom went on to successful astronomical careers of their own. More than five dozen students received their doctorates under Leuschner's guidance.
In 1913 Leuschner became dean of the entire graduate school at Berkeley, and later was appointed head of all World War I related training at the University. He was a founding member of the Astronomical Society of the Pacific, served a term as the president of the American Association of University Professors, and chaired the International Astronomical Union's committee on comets and minor planets for two decades.
Leuschner was one of the first astronomers to dispute Pluto as being Planet X as predicted by Lowell. By 1932 he was already suggesting that Pluto had a mass less than the Earth, and that the discovery of Pluto was an accidental by-product of the Lowell search.
Named after him