Even past his adolescence, Berle continued to advise politicians with his trademark frankness and shrewd understanding of political and economic power. In 1932, Roosevelt relied on him as one of the original members of the "Brain Trust" that forged the New Deal. In the same era, Berle served Mayor Fiorello LaGuardia as City Chamberlain. During World War II, Berle served as Assistant Secretary of State for Latin Affairs; after the war, he acted as ambassador to Brazil.

Berle's career as a lawyer and political advisor was predicated on his scholarship at Columbia. In his seminal works, published while on the faculty of Columbia's law school (1927-1963), Berle identified the emerging power of corporate governors (as opposed to shareholders); he also recognized the government's responsibility for guaranteeing the fluidity and stability of the stock market. His remarkable body of work includes Modern Corporation and Private Property (1933), 20th-c. Capitalist Revolution (1954), Tides of Crisis (1957), and Power Without Property (1959). Through these books and his role as a political consigliere, Berle helped to shape the post-Depression economy from inside an office at the Columbia University law school. Jordan Schwartz, Berle's biographer, explains his significance to the economic planning of his day: "Aspiring to be the Marx of the shareholding class, a great social critic who rallied people to corporate liberalism, he sought to transform the system rather than abolish it — a task he considered as revolutionary as uprooting capitalism itself."