Alexander Wadsworth Longfellow, Jr.

After Richardson's death in 1886, Longfellow teamed up with Frank Ellis Alden and Alfred Branch Harlow to found the firm of Longfellow, Alden & Harlow, with offices in Boston and Pittsburgh. The firm designed the Carnegie Library of Pittsburgh and the City Hall in Cambridge, Massachusetts. They also designed the Arnold Arboretum headquarters, the Hunnewell Building, in 1892, which was constructed with funds donated by philanthropist-horticulturalist Horatio Hollis Hunnewell in 1903.

Longfellow later moved to Boston, where he worked in association with his cousin, William Pitt Preble Longfellow. He designed several structures around Harvard, including the Brattle Theatre, the Phillips Brooks House, the Semitic Museum, the Bertram and Eliot Halls at Radcliffe College, the Robert Stow Bradley Jr. Memorial fountain, and chemical laboratories.

He also designed the Washington Street Elevated, the Theodore Parker Church in West Roxbury, the Merrill Memorial Library in Yarmouth, Maine, the Curtis Memorial Library in Brunswick, Maine, and a Maine Historical Society library building. He served on the board of directors of the Dedham Pottery company and designed their plant.

Longfellow also designed and built Eliestoun, a large shingle-style summer home, rare in the midwest. Eliestoun was completed in 1890 and is on the Principia College campus in Elsah, Illinois.