He was best known for serving as publisher, friend, and mentor of American author Jack London.

Nevertheless, the creation of a separate company in New York was destined to have profound implications for the house of Macmillan, as the American organization outstripped its parent and eventually required complete independence at mid-century.

[10] In 1927, Brett testified at public hearings of the Patents Committee of the United States House of Representatives about a new national copyright law.

[12] During his tenure, Brett published Winston Churchill's novel Richard Carvel in 1899, Ellen Glasgow[13] Upton Sinclair's The Jungle in 1904,[14] Jack London's The Call of the Wild, William Butler Yeats, Liberty Hyde Bailey[15] and Francis Marion Crawford's Saracinesca.

Between 1895 and 1909 such semi-independent branches were established in Chicago, Boston, Atlanta, Dallas, and San Francisco.

"[9] The Bretts remained in control of the American offices of Macmillan from its creation in 1869 to the early 1960s, "a span matched by few other families in the history of United States business.

[23] From 1906 he lived at a 260-acre estate located in the Greenfield Hill section of Fairfield, Connecticut where he maintained an outstanding pinetum on Congress St.[24] The family donated pine land to the city which has been augmented to become the 185-acre conservation area now known as Brett Woods.