Brownell was admitted to the bar in
New York and began his practice in
New York City. In February 1929, he joined the law firm of
Lord Day & Lord in New York, and except for periods of government service, he remained with them until his retirement in 1989. He married Doris McCarter on June 16, 1934. They had four children (Joan Brownell, Ann Brownell, Thomas McCarter Brownell, and James Barker Brownell) and remained together until McCarter's death on June 12, 1979. He married his second wife Marion Taylor in 1987, but the couple separated and divorced in December 1989. His most important client was the famous Greek shipping billionaire
Aristotle Onassis; immediately after the end of
World War II, Onassis was eager to get his hands on the
T2 tankers originally built for the wartime needs of the
U.S. Navy. The tankers were eventually made available for sale, but because they were considered to have a militarily strategic value in the event of another war, they were being offered to American citizens only. Brownell helped Onassis work out a scheme of dummy American corporations, thus allowing him to bypass the regulations and purchase the tankers through these
dummy corporations. Later, as Attorney General, Brownell would be forced to switch sides under pressure from FBI Director
J. Edgar Hoover, and his Justice Department would
indict Onassis (eventually Onassis and the U.S. government reached a settlement). ==State political career==