Arnolds family moved to Ludlowville, New York and he attended Cornell University with the intent to study agriculture. He received his Bachelor of Science at in 1924, his Doctor of Philosophy in 1929 with his thesis on Devonian megafloral paleobotany.
Interaction with Loren Petry, a Cornell professor studying Devonian plants of the region, lead to Arnold shifting his focus to paleobotany. He started working at the Faculty of Botany, University of Michigan from 1928 and became curator of the collection of fossil plants in 1929. Arnold became a professor in 1947.
Arnold did extensive research on the flora Paleozoic, Mesozoic and Tertiary of North America studying fossils from British Columbia to Oklahoma to Greenland.
During his lifetime Arnold wrote approximately 121 publications, on subjects including the fossil conifers of Princeton, British Columbia to the extinct water-fern, Azolla primaeva. A number of fossil plants have been named in Arnold"s honor including Koelruteria arnoldi and Pseudolarix arnoldi.
Arnold interacted with a number of eminent profession and amateur paleobotanists across the western United States. While collecting fossils with Alonzo West. Hancock in the Clarno Formation of Oregon in 1941, Arnold and Hancock recovered the most complete Miomastodon skull known to date. In 1952 Arnold was the supervisor for Herman F. Becker who extensively studied the Ruby Basin Flora of Montana.
Among the may correspondents of Arnold was Wesley C. Wehr, who became Affiliate curator of Paleobotany at the Burke Museum of Natural History and Culture in Seattle.
Arnold was a member of many learned societies and was the author of the Introduction to Paleobotany published in 1947.