Dr. Callender was born near Nashville, Tennessee, and received his college education at the University of Nashville. In 1851, he entered the law department at the University of Louisville, and the following year, he went to Harvard to continue his law studies. His father’s illness called him home, and his legal studies were abandoned. He began to study medicine and received his M.D. from the University of Pennsylvania in 1855.
He owned and edited the Nashville Patriot newspaper for three years, and in 1858, he was elected Chair of Materia Medica and Therapeutics at Shelby Medical College in Nashville. In 1861, he was appointed surgeon to the 11th Tennessee Confederate Regiment during the Civil War. In 1869, he was appointed superintendent of the Tennessee State Hospital for the Insane, where he remained until 1895. He resigned with plans to open a private sanitarium, but he died the following year.
Dr. Callender was Chair of Diseases of the Brain and Nervous System at the University of Nashville and, in 1880, Professor of Physiology and Psychology at Nashville and the affiliated Vanderbilt University. At the 9th International Congress in Washington in 1887, he was elected President of the Section on Physiology and later served as Dean of the Medical Faculty at Nashville. He was President of the Association of Medical Superintendents of American Institutions for the Insane (1882–1883).