Longfellow was graduated from Harvard University in 1876 and later studied architecture at the Massachusetts Institute of Technology and the Ecole des Beaux-Arts in Paris. His professional affiliations were in Boston, Massachusetts. The firm of Longfellow, Alden & Harlow designed the Carnegie Library in Pittsburgh, Pennsylvania and the Cambridge, Massachusetts City Hall. Later in association with his brother, he designed many New England structures, including Phillips Brooks House, Semitic Museum, and chemical laboratories at Harvard University as well as dormitories at Radcliffe College. He was founder and former president of the Marine Museum, a trustee of the Boston Museum of Fine Arts and the Boston Athenaeum, and a member of the Boston Art Commission. He was for many years an officer of the Boston Society of Arts and Crafts.