[1][2] He was the son of Susan Allen (née Marple) and Thomas Corwin Mendenhall.

[1][3] Starting in 1895, he studied under Henry Rowland at Johns Hopkins University and received a PhD in 1898.

[1][2][4] Under Rowland, he worked with Charles Greeley Abbot, head of the Smithsonian Astrophysical Observatory, and fellow student Frederick A. Saunders, a fellow PhD candidate, on a black-body radiation problem for his thesis.

[3] After graduation from Rose Polytechnic in 1894, Mendenhall worked with George Putnam to make a transcontinental survey of the acceleration of gravity for the United States Coast and Geodetic Survey and taught physics for a year at the University of Pennsylvania.

[1][3] In 1901, he succeeded fellow Hopkins graduate Robert W. Wood as assistant professor at the University of Wisconsin–Madison.

[1] In 1917, Mendenhall was made a Major of the Science and Research Division of the U.S. Army Signal Corps.