Rosenberg's reputation for clean dealing enabled him by 1935 to persuade his brother-in-law Jacques Helft, a noted antiques dealer, to open a branch in Bond Street, London, to enable them to engage with more Americans. Noted clients included museums such as the Museum of Modern Art (MoMA) (of which Rosenberg was an early supporter and donor), and the Philadelphia Museum of Art. His private clients included Alfred H. Barr, Jr., Chester Dale, Douglas Dillon, and Marjorie and Duncan Phillips, who through purchases from Rosenberg created much of the modern collections within The Phillips Collection in Washington, D.C..