Leonard was born in Mount Vernon, Indiana in 1896 and moved with his family to Chicago in about 1900, eventually settling near the University of Chicago. From the age of eight, he showed great interest in the stars and by early adolescence had become an active amateur astronomer. In 1909 he attended the annual meeting of the Astronomical and Astrophysical Society of America, held at the Yerkes Observatory. The same year, he organized the Society for Practical Astronomy (SPA), a national amateur organization. Leonard's leadership raised concerns among professional astronomers as not all were in favor of amateur contributions to the profession. Nonetheless, the organization flourished until Leonard's departure in 1919.