He spent two summers at the Michigan University Biological Station, then in 1925, he started as a botanist at the US Department of Agriculture, serving under the USDA's chief agrostologist A. S. Hitchcock and after Hitchcock's sudden death in 1935, Agnes Chase.
Swallen practiced botany in California in 1927, and from the southwest United States to Yucatan, Mexico, in 1928, 1931 and 1932.
In 1947, he became the curator of the Division of Grasses at the Smithsonian Institution and chaired the Botany Department from 1950 until his retirement in 1965.
He was married a second time, to Clara Bayne Brasel, a 1941 Graduate of George Washington University and a Secretary at the Smithsonian, on August 28, 1955.
The genus Swallenia for Eureka Valley dune grass, endemic to Inyo County, California, commemorates his name.