He was the son of Alexander Wadsworth Longfellow Sr. (1814–1901), a United States Coast Survey topographer, and the former Elizabeth Clapp Porter.

After graduating from Harvard University in 1876, he studied architecture at the Massachusetts Institute of Technology and the École des Beaux-Arts in Paris, and then worked as senior draftsman in Henry Hobson Richardson's office.

The firm designed the Carnegie Library of Pittsburgh and the City Hall in Cambridge, Massachusetts.

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.

[4] Longfellow also designed and built Eliestoun, a large shingle-style summer home, rare in the midwest.