Slonimsky immigrated to the United States in 1923 to work as an accompanist in the newly formed Opera Department at the Eastman School of Music in Rochester, New York, where he continued his composition and conducting studies. After two years, he moved to Boston, Massachusetts to work as an assistant for Boston Symphony Orchestra conductor, Serge Koussevitsky, for whom he had earlier worked as a rehearsal pianist in Paris. During this time, Slonimsky taught music theory at Boston Conservatory and the Malkin Conservatory and began to write music articles for The Boston Evening Transcript, The Christian Science Monitor and the magazine, The Etude.
In 1927, Slonimsky formed the Boston Chamber Orchestra, for which he solicited music from contemporary composers. Slonimsky was a great champion of contemporary music. He conducted the world premieres of Edgard Varèse's Ionisation for thirteen percussionists in 1933; of Charles Ives' Three Places in New England in 1931; and various other works.
In 1958, Slonimsky took over the supervision of Baker's Biographical Dictionary of Musicians and worked as head editor until 1992. He also wrote Music Since 1900, a survey of almost every important musical event in the 20th century and The Lexicon of Musical Invective, a compilation of hilariously bad reviews by critics of composers since Beethoven's time. One of his best-known books is the Thesaurus of Scales and Melodic Patterns (ISBN 0-8256-1449-X), which has influenced many jazz musicians and composers (including Frank Zappa, John Coltrane, composer John Adams and guitarist Buckethead).
During 1986, Slonimsky made frequent Saturday afternoon visits to appear on the Doug Ordunio show, heard over KFAC-FM, Los Angeles. During one of the shows, a crew came out from the New York public television station, WNET, to film the entire show. Portions of this session were included in the "Aging" segment of the PBS Series The Mind. Slonimsky possessed a sly sense of humor, a trait he would exhibit on the various appearances he made on The Tonight Show with Johnny Carson. Late in life, he became a good friend of avant-garde rock guitarist and composer, Frank Zappa, and performed some of his own compositions at a Zappa concert in Santa Monica, California in 1981. Slonimsky named his cat Grody-to-the-Max after the learning the phrase from Zappa's daughter, Moon-Unit.
Slonimsky died at age 101.
Slonimsky's notable students include Cynthia Tse Kimberlin. He left a superb autobiography, Perfect Pitch (ISBN 0-1931-5155-3), filled with lively anecdotes about a great range of music figures of the 20th century, including his mentor, Serge Koussevitzky, Charles Ives, Henry Cowell, Igor Stravinsky, and many others.