Born in South Boston, Gallagher studied horticulture at Harvard’s Bussey Institute, supplemented by classes in the Fine Arts program where he met Frederick Law Olmsted, Jr. Graduating in 1894, Gallagher joined Olmsted, Olmsted, & Eliot. His projects there included restoration of the plants on the U.S. Capitol Grounds in Washington, D.C. In 1904, he attempted to open a firm with landscape architect James Sturgis Pray who later became the chairman of the Department of Landscape Architecture at Harvard. Gallagher was ill-suited for the administrative side of running a firm, however, and returned to the Olmsted office after two years, at which point his former firm became Pray, Hubbard, & White.
In 1927, he became a full partner with Olmsted Brothers. Noted for his artistic talent, horticultural knowledge, and generally modest temperament, Gallagher was well suited to collaborative work with colleagues and clients alike. He contributed to the design work for Bryn Mawr, Haverford, Swarthmore, Vassar, and Duke University. Gallagher worked on the design of the park system for Union County, New Jersey, including Rahway River Park, and his most extensive estate work took place on Long Island, including Ormston, the George Baker estate, and the H.H. Rogers estate. These latter projects show a range of plant palettes and a facility with multiple design styles. His ease with stylistic juxtapositions is well shown at the Oldfields estate in Indianapolis, now part of the Indianapolis Museum of Art.