During his time at Root, Clark & Bird (which subsequently became Dewey Ballantine), he was also one of the architects of modern Wall Street's legal culture.

He first gained prominence in the district attorney's office when he was chosen to conduct a highly publicized investigation into corruption allegations at the New York Police Department.

Recognized as one of the greatest trial lawyers of his time, Buckner was appointed by President Calvin Coolidge to the post of United States Attorney for the Southern District of New York.

[1] Soon after his appointment, Buckner established his independence from the Justice Department by ending the use of special antitrust attorneys who reported to officials in Washington.

The New York Times reported that “his most outstanding work has been in connection with liquor cases and his several ‘padlock drives’ which temporarily closed hundreds of resorts where liquor was being sold illegally.”[5]  He also received national attention when he insisted on prosecuting Harry M. Daugherty (United States Attorney General under Warren G. Harding) for public corruption, against the wishes of the Coolidge administration.

He also initiated the practice of having an annual luncheon with hiring partners at the other Wall Street firms, at which they set the "going rate" for associate salaries.

Buckner explored the possibility of leveraging the notoriety he gained as US Attorney into a run for governor of New York, but reconsidered after he suffered a stroke in 1934.

During this period, his chief assistant at Root, Clark was John Marshall Harlan II, who would later serve with Buckner's friend Frankfurter on the United States Supreme Court.