[1] He began working in business for Cushman Bros., which later became Lamont, Corliss, and Company, and turned it into a successful importing and marketing firm.

In 1918, he purchased the New York Evening Post, of which his brother, Hammond, had been managing editor a decade earlier, from Oswald Garrison Villard.

He also served unofficially as an advisor to a mission to the Allies, led by Edward M. House, as requested by President Woodrow Wilson.

He and the head of the American Red Cross, William B. Thompson, along with the approval of the British prime minister, Lloyd George, tried to convince America to aid the Bolsheviks so that Russia would stay in the war.

Both he and Norman H. Davis were appointed as representatives of the Treasury Department to the Paris Peace Conference and had to determine what Germany had to pay in reparations.

He handled the press and defended the firm during hearings like those of Arsene Pujo that investigated powerful Wall Street bankers.

A member of the Council of Foreign Relations, he was an unofficial advisor to the Wilson, Herbert Hoover, and Franklin Roosevelt administrations.

However, he did not aggressively challenge Japanese efforts to build a sphere of influence in Manchuria;[8] indeed, he supported Japan's non-militaristic politics until late into the 1930s.

Lamont was the chairman of the International Committee of Bankers on Mexico for which he successfully negotiated the De la Huerta-Lamont Treaty.

[3] On September 20, 1940, the fascist police shocked Lamont by arresting Giovanni Fummi, J.P. Morgan & Co.'s leading representative in Italy.

On Black Thursday in 1929, Lamont was acting head of J.P. Morgan & Co. Five days prior to the Crash, President Herbert Hoover had contacted Lamont with concerns about the rampant market manipulation by Wall Street insiders, and the systemic risk it presented to the stock market.

At the end of World War II, Lamont made a very substantial donation toward restoring Canterbury Cathedral in England.

His widow, Florence Haskell Corliss donated Torrey Cliff, their weekend residence overlooking the Hudson River in Palisades, New York, to Columbia University.

Another son, Thomas Stilwell Lamont, was later vice-chairman of Morgan Guaranty Trust and a fellow of the Harvard Corporation.

Lamont is a major character in Nomi Prins' novel Black Tuesday (2011) and in Kit Holland's Soul Slip Peak (2013).