Until the mid 1930s Sessions’ music was neo-classical. But although Sessions believed in the ideal of a single Western musical culture and thus maintained a conservative vision of American music, after the mid 1930s his rhythms became more complicated, full of chromatic, dissonant harmonies, often combined with a dense and quasi-improvisatory character, and, after 1953, dodecaphony. As Elliott Carter described it: "More and more the notion of extended, continuously flowing sections during which ideas come to the surface, gain clarity and definition, and then sink back into the general flow has characterized Sessions’s unique style." (Current Chronicle, The Musical Quarterly, July 1959).

Among his works are a "Mass", some operas (e.g.: "Montezuma", 1963/64), cantatas (e.g.: "When Lilacs Last in the Dooryard Bloom’d", 1971), a Violin Concerto (1935), piano music (e.g.: a Piano Concerto or the Piano Sonata No. 3, 1964/65, his last one), a "Concertino for Chamber Orchestra", "Six Pieces for Violoncello" (1966), a String Quintet (1957/58), two String Quartets (e.g.: No. 1, 1936) and some Canons for String Quartet.

Among his orchestral works are 9 symphonies (e.g.: No. 1, 1927; No. 2, 1946; No. 3, 1957; No. 4, 1958; No. 5, 1964) and a "Rhapsody" (1970).