By the late 1920s McMillen Inc. had become a full-service decorating business with an independently-staffed drafting department as well as a business office. Jobs completed by McMillen before the Great Depression included a Hunting Box in Southampton, New York, for Colonel H.H. Rogers of the Standard Oil fortune, a suite of rooms on Commonwealth Avenue in Boston for Mrs. Louis A. Frothingham, and a Salon in the Restoration Style for a 1927 Exhibit at the Grand Central Palaces in New York City.
For the most part, these commissions might be described as 'historicist' in style, whether in the Americana vernacular, like the Port of Missing Men, or in the Louis XV taste, such as the Lorrillard's drawing room. In particular, Empire and Directoire motifs and furniture were frequently used by Mrs. McMillen during these years. Some of McMillen's rooms in the country, such as the house for Mrs. Hubert McDonnell in Greenwich, the house for Ambassador and Mrs. Sheldon Whitehouse in Newport, or the Studio on the beach in Southampton for Archibald Manning Brown, were treated in a more casual, modern and eclectic manner.