O'Donnell, along with Larry O'Brien and David Powers, was part of the group of Kennedy's close advisers dubbed the "Irish Mafia.

[3] He was the son of Alice M. (Guerin) and Cleo Albert O'Donnell, who was the football coach at the College of the Holy Cross Crusaders for two decades, and later athletics director for all sports activities.

[5] O'Donnell graduated from high school during World War II and then served in the U.S. Army Air Forces (1942–1945), where he flew 30 missions as a bombardier in a B-17 squadron before being shot down over Belgium.

"He was imprisoned, escaped, and emerged with the Distinguished Flying Cross and Air Medal with Four Oak Leaf Clusters.

[10][11] In their memoir of Kennedy, Johnny, We Hardly Knew Ye, both O'Donnell and David Powers reported hearing only three shots and did not offer any speculation as to their origin.

[12] According to a June 15, 1975 report in the Chicago Tribune, an unnamed "Central Intelligence Agency liaison man" told Congressmen that O'Donnell and David Powers had initially told assassination investigators that the shots that struck Kennedy came from a location other than the Texas School Book Depository, but that the two men were convinced, reportedly by FBI Director J. Edgar Hoover or his top aides, to alter their accounts to the Warren Commission to avoid the possibility of revealing the CIA's plots to kill Fidel Castro which might lead to an international incident.

[14] O'Donnell's son, Kenneth Jr., stated that his father privately called the Warren Commission “the most pointless investigation I’ve ever seen", and that he claimed shots came from two different directions.

[15] After serving as a presidential aide to Lyndon Johnson until early 1965,[7][16] O'Donnell resigned in order to try to win the Democratic nomination for Governor of Massachusetts in 1966.

[8] On August 11, 1977, O'Donnell was admitted to Beth Israel Hospital in Boston for a gastrointestinal ailment brought on from the effects of alcoholism.

[24] In his biography With Kennedy (1966), Pierre Salinger writes: It was my impression that O'Donnell had the greatest influence in shaping the President's most important decisions.

He was able to set aside his own prejudices against individuals and his own ideological commitments (I would rate him a moderate Democrat) and appraise the alternatives with total objectivity.

It was impossible to categorize O'Donnell, as White House observers did with other staff members, as either a "hawk" or a "dove" on foreign policy, or a Stevenson liberal or Truman conservative on civil rights.

[citation needed] In 1998, William Morrow & Co. published A Common Good: The Friendship of Robert F. Kennedy and Kenneth P. O'Donnell.