After an interruption for service in the U.S. Navy during World War I, he was elected to the Elihu Senior Society and graduated in 1920.

[3] From 1924 to 1926, Auchincloss practiced law in New York City, before joining the Commerce Department as a special agent in aeronautics.

At the time of the merger, the new firm, known as Thomson & McKinnon Auchincloss, had assets under management of $160 million (equivalent to $856,702,000 in 2023) and 58 offices.

They remained married until his death in 1976 and had two children together: Auchincloss was responsible for getting Jacqueline Bouvier her first job in journalism at the Washington Times-Herald.

He gave her away at her wedding to future president John F. Kennedy, the reception of which was held at Hammersmith Farm on September 12, 1953.

[11] A long-time financial contributor to the Republican Party, he contributed to the campaign of his Democratic stepson-in-law, saying "I want to live in harmony with Mrs. Auchincloss and all the other members of the family."