On January 7, 1866, James was born to Isaac Mosier and Rebecca Penrose Gidley in Springwater, Iowa. James attended Princeton University, earning a BS degree in 1898 and an MS degree in 1901. Years later he attended George Washington University, where he earned a PhD degree in 1922. He married Florence Emily Martin on April 4, 1900.
James was a vertebrate paleontologist whose first professional work was with the American Museum of Natural History in New York, from 1899 to 1905. After that he took a position at the National Museum of Natural History and moved to Washington, D.C., and in 1911, he became the assistant curator of fossil mammals. He conducted many expeditions to fossil sites in the United States while working for both of these museums.
James was elected to the Washington Biologists’ Field Club in 1908 and his membership ended in 1917.
He died on September 26, 1931.