[2][3][4] MacDougal was employed by the United States Department of Agriculture to collect specimens in Arizona and Idaho during the summers of 1891 and 1892.

[2][4] He attended the lectures, in United States, by Dutch botanist Hugo de Vries on Mutation Theory.

In 1905 he helped published Hugo de Vries's lectures into a book Species and Varieties: Their Origin by Mutation.

In 1905 he was involved in the establishment of the Plant Desert Laboratory in Tucson, Arizona, where he continued his experiments for several years with breeding studies on the Oenothera (evening primrose).

[2][4] In 1909, MacDougal established the Coastal Laboratory at the Outlands in the Eighty Acres in Carmel-by-the-Sea, California, where he conducted research on the Monterey pine (Pinus radiata).

His book Growth in Trees was published in 1921 and encompasses 18 years of research in Arizona, California, Idaho, Mexico and the Libyan Desert.

In 1914, he wrote the book The Salton Sea: A Study of the Geography, the Geology, the Floristics, and the Ecology of the Desert Basin.

He was a member of several scientific organizations, including the Koninklijke Hollandsche Maatschappij der Wetenschappen and the American Academy of Arts and Sciences.