Ellsworth (“Buddy”) was born on September 2, 1890, in Rochester, New York. He completed an AB degree at the University of Rochester in 1911 and worked as an associate curator at the Rochester Academy of Science until 1917. He was in military service from 1918 to 1919, during which time he won the Chevalier of the French Legion of Merit. After the War, he was appointed an aide in the Division of Plants at the Smithsonian Institution and became head curator in 1946 after the retirement of W. R. Maxon. Ellsworth retired in 1950 at the age of 60.
Ellsworth was a avid collector and collected widely in the United States especially in Florida (1935-40, 1950-56) and in Colombia (1917-48). He also collected in Argentina, Brazil (1929), Chile, Cuba (1931, 1937), Jamaica (1916, 1920), Panama (1917-18, 1948), Peru (1929), and Venezuela (1943). He often accompanied other botanists such as A. C. Smith, E. C. Leonard, W. R. Maxon, F. W. Pennell, and P. C. Standley. He specialized in Bomarea (Amaryllidaceae) as well as Leguminosae and Passifloraceae, and his name is attached to more than 600 species names. About 150 species are named for him. He was a corresponding member of the Venezuela Natural Science Society.
He died on November 21, 1968, in Redlands, California.
Ellsworth was elected to the Washington Biologists’ Field Club in 1922, served as president from 1934 to 1937, and was awarded an honorary membership in 1961. He appears in many Club photographs, such as those of removing the invasive Japanese honeysuckle and improving the cabin and grounds.