Phelan was born in Bridgeport, Connecticut and received his architectural training from the Pratt Institute in Brooklyn, New York, (class of 1913) after which he worked for two years as a designer for A. C. Kelley and Co, Bridgeport, Connecticut.

He joined the Bridgeport, Connecticut engineering firm of Fletcher Thompson in 1916 as a draftsman and worked his way up to managing principal in 1931.

[1] Pratt Institute's J. Gerald Phelan Award for Architecture is named in his honor.

[2] In 1969 Phelan was awarded a Doctor of Arts honorary degree from Fairfield University for whom he had designed 10 buildings over the years.

Phelan was a longtime member of the American Institute of Architects (AIA) and served as the president of the Connecticut Chapter between 1942 and 1943.