Born in Somerville, Massachusetts, O'Donnell was the son of a doctor and pursued his education at Tufts College, graduating in 1920, followed by further studies at Harvard University.
In 1927, he joined the New York Daily News, where he gained recognition for his political journalism, covering significant events such as Franklin D. Roosevelt's 1933 presidential inauguration and serving as a correspondent during World War II, reporting from the Maginot Line in France.
Throughout World War II, O'Donnell's journalism reflected the right-wing political biases of his publisher, William Randolph Hearst, which included shifting from initial admiration to harsh criticism of President Roosevelt.
O'Donnell later reported on allegations regarding sexual promiscuity in the Women's Army Corps and also an antisemitic conspiracy theory about General George S. Patton's removal.
In 1939, after World War II broke out, he became a correspondent for the Daily News; in this capacity he spent time with the British military's forces on the Maginot line in France.
[5] In June 1943, O'Donnell's Capitol Stuff column did "incalculable damage" to the Women's Army Corps,[6] thwarting recruiting efforts in war time.
"[9] This followed O'Donnell's June 7 column discussing efforts of women journalists and congresswomen to dispel "the gaudy stories of the gay and careless way in which the young ladies in uniform … disport themselves.
"[18] On October 3, 1945, O'Donnell wrote in his column that Gen. George S. Patton had been removed from his Army command in Bavaria thanks to "the secret and astoundingly effective might of this republic's foreign-born political leaders — such as Justice of the Supreme Court Felix Frankfurter of Vienna, White House administrative assistant Dave (Devious Dave) Niles, alias Neyhaus, and the Latvian ex-rabbinical student now known as Sidney Hillman."