Draftsman, G. Vincent Palmer, Architect, Los Angeles, CA, 1935; Draftsman, Russell E. Collins, Architect, Los Angeles, CA, 1940-1941; Designer, Ralph C. Flewelling, Architect, Los Angeles, CA, 1946-1948; Partner, Neptune and [John S.] Gregory, Architects and Engineers, Pasadena, CA, 1948-1953; Partner, Neptune and Thomas, Architects, AIA, Pasadena, CA, 1953- ;

Fellow, American Institute of Architects (FAIA).

A.B., University of California, Berkeley, Berkeley, CA, 1940. His brother, Kenneth, graduated from UC Berkeley six years earlier.

Delta Sigma Chi, University of California, Berkeley, Berkeley, CA, 1940;

The Neptune Family lived at 3994 Alabama Street, San Diego, CA. Their house was mortgaged at this time, and several members of the extended family lived with them; Donald's maternal grandparents and his aunt both resided with the family in 1920. In 1930, Neptune continued to live with his family in San Diego at 4448 Marlborough Avenue. The family owned their own house in 1930.

His father, Herman P. Neptune (born c. 1883 in WV), worked in 1920 in real estate, and ten years later registered with the US Census as a residential carpenter. Herman's parents both came from OH. Kenneth's mother was Frances J. Harper (born c. 1885 in MN). Her father, Erhard (born c. 1852), had been born in PA, her mother, Elizabeth M. Harper (born c. 1867), in England. Frances may have lived for some time in CO, like her husband. Herman and Frances married c. 1910 and had 3 sons at home in 1930: Kenneth H., the eldest, (born 06/12/1911 in Boulder, CO) and Homer E. (born c. 02/1918 in CA). Both Donald and Kenneth became architects and worked in Southern CA for their whole careers.

In his later years, Neptune lived and worked in Orange County, CA.