He researched number theory; see in particular Bell series. He attempted—not altogether successfully—to make the traditional umbral calculus (understood at that time to be the same thing as the "symbolic method" of Blissard) logically rigorous. He also did much work using generating functions, treated as formal power series, without concern for convergence. He is the eponym of the Bell polynomials and the Bell numbers of combinatorics. In 1924 he was awarded the Bôcher Memorial Prize for his work in mathematical analysis. He died in 1960 in Watsonville, California.