American physicist and meteorologist, the first to propose the use of a ring pendulum for measuring absolute gravity.

Mendenhall was a professor at Ohio State University, Columbus, in 1873–78 and from 1881 until he was named professor emeritus in 1884, when he became a professor for the U.S. Signal Corps. In 1878–81 he was a visiting professor at the Tokyo Imperial University, where he helped develop the Japanese government's meteorological system, and in 1889 he was appointed superintendent of the U.S. Coast and Geodetic Survey; he served as president of the Worcester (Mass.) Polytechnic Institute from 1894 until 1901, when he moved to Europe for 11 years of work.

Mendenhall developed the state weather service of Ohio, and his work included studies in electricity, seismology, and atmospheric electricity.