[1] An only child, he was stricken by pleuropneumonia at age 15, and to aid his recovery, was ordered by his physician to spend his time out of doors walking and being active.

For several years following his illness his activities included frequent visits to the Arnold Arboretum, where he gained the acquaintance of professor F. H. Storer, agricultural chemist and Dean of Harvard's Bussey Institution who influenced Hosmer's development.

[2] Hosmer attended the Bussey Institution and the Lawrence Scientific School of Harvard University from Fall 1891 to June 1895, graduating with a bachelor's degree in Agricultural Science.

After completing his degree at Yale in 1902, Hosmer returned to Washington and was promoted the following year to Chief of the Section of Forest Replacement in the Bureau of Forestry.

His contributions are memorialized at the Ralph S. Hosmer Grove in Haleakala National Park, which is now the site of a campground and picnic area.

In 1914, Hosmer replaced Walter Mulford as Professor and head of the Department of Forestry at the New York State College of Agriculture at Cornell University, a position he held until his retirement in June 1942.