Rowell was appointed librarian of the University of California in 1875, the year following his graduation. He served in that capacity for forty-four years, during which time the collection grew from 13,000 volumes to 400,000. The acquisition of the Bancroft Collection, earnestly pursued by him in 1898, and supported again in 1905, was but one of the many contributions he made to the establishment of the Berkeley library as a major research center.