Smallens was born in Saint Petersburg, Russia, and emigrated to the United States, as a child, becoming an American citizen in 1919. He studied at the New York Institute of Musical Art until 1909, when he traveled to France to study at the Conservatoire de Paris.
Returning to the United States, Smallens was a conductor or music director at several American music organizations including the Boston Opera Company (1911–1914), the Anna Pavlova Ballet Company (1917–1919), the Chicago Opera Company (1919–1923), the Philadelphia Civic Opera (1924–1931), the Philadelphia Orchestra (1928–1934) and the Radio City Music Hall (1947–1950).
In addition, Smallens worked briefly on Broadway, conducting the premieres of Thomson's Four Saints in Three Acts in 1934 and Gershwin's Porgy and Bess the next year. (Both works were operas, not the musicals normally expected in Broadway theatres.) Smallens also conducted the Porgy and Bess revivals on Broadway in 1942 and 1953, as well as the famous 1952 world tour of the work, which culminated in that 1953 Broadway production.
Smallens also conducted orchestras for music as part of several documentary films in the late 1930s and early 1940s. He retired from music in 1958 and moved to Sicily. Smallens died in Tucson, Arizona in 1972, where he is buried.