George Lee Leslie was born in August 1858 in Lima, Wisconsin. He later lived in Muscatine, Iowa. Leslie attended the University of Illinois, from 1879 to 1880. He received his B.S. degree in 1882 and M.A. degree in 1885, both from the State University of Iowa. While earning his master's degree, he was head of science instruction at Iowa City High School. After receiving his master's degree, he served as principal of public schools in Sheffield, Illinois and in Luverne, Minnesota, until 1891, when he moved to Santa Barbara, California. He was head of science instruction at the Santa Barbara High School through 1893. Leslie served as high school principal at Rock Island, Illinois, from 1894 to 1895.

In 1895, Leslie returned to California to serve as head of the science department in the Los Angeles High School. In 1899 he taught science courses at the San Francisco State Normal School for one year before returning to his position in the Los Angeles school system. In Los Angeles he was promoted to director of the Health and Psychology Department within the city school system, a position he held from 1906 to 1914. While working as director, Leslie helped draft a bill that became California law in 1909, which provided for health and development supervision in the public schools.

In 1921, Leslie wrote a letter to the California Legislature in support of incorporating a technical institution with the University of California. According to Leslie, the bill passed both the Senate and the General Assembly, but was vetoed by Governor William Stephens. In 1921, the Throop College of Technology, which was established in 1891 by Amos G. Throop, was renamed the California Institute of Technology (referred to as Caltech).

Professor Leslie was married and they had a daughter, Virginia. George L. Leslie died in 1929.