Walter Mellor was born in Philadelphia, the son of Alfred and Isabella (Latham) Mellor. He graduated from the Haverford School in 1897 and received his B.S. in Mechanical Engineering in 1901 from Haverford College, before going on to the University of Pennsylvania and gaining his B.S. in Architecture in 1904. After graduation he entered the firm of T. P. Chandler
, where he became acquainted with Arthur I. Meigs
, a graduate of Princeton University. In 1906 the two men established the partnership of Mellor & Meigs
, with offices in the Lafayette Building in Philadelphia. Almost immediately successful, Mellor & Meigs became well-known for their designs of clubs and private residences. Their early work included the more modest homes developed by the Lower Merion Realty Co., as well as large-scale undertakings such as alterations to the Pickering Hunt Club in Phoenixville, PA (1911), and the Princeton Charter Club in Princeton, NJ (1913). Familial and social association played a large role in the clientele of Mellor & Meigs, adding several fraternity houses for Phi Gamma Delta, Mellor's fraternal affiliation, and academic buildings for Haverford College, Mellor's alma mater, as well as residences for the Meigs and Browning families.
In 1916 Mellor & Meigs were joined by George Howe, fresh from the venerable firm of Furness, Evans & Co. Since World War I almost immediately drew Howe away from the office for some three years, his influence cannot be said to be felt before 1919; and his designs for the Philadelphia Saving Fund Society branch offices were the only truly commercial work attempted by the firm. It is not surprising, therefore, that when Howe seceded from the office in 1928, taking with him the PSFS account and later designing the International Style PSFS Building at 12th and Market streets in Philadelphia, the partners simply continued to design the country houses for which they had earlier achieved a reputation.
In 1922 the firm won the Philadelphia Chapter, AIA, award for the McCracken residence in Germantown, Philadelphia, and in 1925 the Gold Medal of the Architectural League of New York for the Newbold Estate development.
Mellor was a fellow of the AIA and a member of the local chapter in addition to holding membership in the Philadelphia Art Alliance, the Union League, the Philadelphia Zoological Society (for whom the firm designed the Bird House), the Germantown Cricket Club, and the Mask & Wig Club. He was in addition a director of the Kestner Evaporating Co. and a trustee of the Cummington School in Massachusetts.