[1] Gallagher was born April 30, 1869 in South Boston to parents who were members of the city's mercantile class; his father was a cabinetmaker and stove merchant and his mother was a descendant of Massachusetts Bay Colony pilgrims.

[7][8] Gallagher also received instruction, especially in the techniques of etching, from Charles H. Woodbury, an accomplished and successful artist who founded the Ogunquit School of Art in Maine.

[10] After Juglaris returned to Italy, Gallagher began studying with Samuel Peter Rolt Triscott, an English-born painter with a growing reputation in Boston.

[5] In April 1895, Gallagher married Charlotte Dodge and shortly thereafter left on a honeymoon to Europe, beginning in England but with the apparent goal of settling in Paris for further instruction.

[17] Like such other Americans as Woodbury, F. W. Benson, Edmund Tarbell, and Childe Hassam, Gallagher joined the ateliers of Jean-Paul Laurens and Jean-Joseph Benjamin-Constant at the Académie Julian and settled in to study and paint.

[25] Gallagher enjoyed early success, both financially and in popular response, and through the 1920s and 1930s he occupied a prominent position in the American art world, being favorably compared to artists like Woodbury, Benson, and even Winslow Homer (1836–1910).

The influences of impressionism on his art may be seen in his frequent use of animated and summary brushwork to express transitory aspects of nature and in his tendency to incorporate the tone of his paper’s reserves in a work as means of adding the sparkle of sunlight to a scene.

[6] Gallagher's low visibility might be explained by the very fact of his prolific output; he produced a quantity of work that may have diminished the relative value of individual paintings and earned him the label of being a "popular" artist rather than a master.

It may also be that the large output necessitated repetition in subject matter and style, resulting in so many versions of similar scenes (for example, those of waves and rocks at Monhegan Island) that few attained individual recognition.

That he produced beautiful landscapes and seascapes, delightful book and magazine illustrations, and sharply defined etchings should be enough to draw attention to this successful artist, a notable American impressionist.