[1] Although he supported Theodore Roosevelt in the 1912 presidential election, O'Mahoney switched to the Democratic Party the same year he joined the State Leader.

[3] Governor Kendrick became a U.S. senator in March 1917, and O'Mahoney accompanied him to Washington, D.C. as his executive secretary, a position he held for three years.

[5] He was a delegate to the 1932 Democratic National Convention in Chicago, Illinois, where he was a member of the subcommittee which prepared the party's platform.

As a reward for his work at the 1932 convention,[6] O'Mahoney was appointed by Farley to be the First Assistant Postmaster General, serving from March to December 1933.

[5] During his early tenure in the Senate, O'Mahoney supported most of the New Deal programs, with the notable exception of President Franklin D. Roosevelt's "court-packing plan".

[6] He earned a reputation as a strong opponent of big business and monopolies, and was heavily involved with anti-trust legislation.

[5] O'Mahoney was among twelve nominated at the 1944 Democratic National Convention to serve as Roosevelt's running mate in the presidential election that year.

[8] In 1952, as Republican Dwight D. Eisenhower won the presidential election in a landslide, O'Mahoney was narrowly defeated for re-election by Governor Frank A. Barrett by a margin of 52% to 48%.

[4] He sponsored legislation to require "concentrated industries" to give public notification and justification of price increases, to prohibit automobile manufacturers from operating finance firms, to grant Alaska and Hawaii statehood, and to require nominees for federal judgeships take an oath prior to confirmation that they would not render decisions contrary to the U.S.