McKeon was defended by colorful civilian attorney Emile Zola Berman. Berman would later go on to defend Sirhan Sirhan. Berman put on a vigorous defense that swayed both the court and public opinion. Marine Corps Commandant General Randolph Pate testified. One reporter pointed out this was like "calling J. Edgar Hoover to testify about a problem within the FBI". The trial's most dramatic moment, however, was the arrival of General Lewis "Chesty" Puller, the most decorated Marine in the history of the Corps. Berman called Puller to testify about training methods. Although having some very harsh private words for McKeon, Puller called the incident in Ribbon Creek "a deplorable accident", but one that did not warrant court martial. He said that discipline was the most important factor in military training. He quoted Napoleon in saying that an army becomes a "mob" without it. He mentioned his experiences in the Korean War and one of the reasons troops failed was because of lack of night training. General Puller felt that the press had blown this incident out of proportion because of prejudice they had against the Marine Corps. He mentioned a similar accident at an Army post where ten soldiers drowned and pointed out that none of their superiors had been charged and that it had never made headlines the way Ribbon Creek did.
In the end, McKeon was acquitted of manslaughter and oppression of troops. He was found guilty of negligent homicide and drinking on duty. The sentence was a $270 fine, nine months of confinement at hard labor, rank reduced to private and a bad conduct discharge.
The Secretary of the Navy later reduced the sentence to three months in the brig, reduction to private with no discharge and no fine. McKeon went back on active duty. He was never able to regain his former rank and was forced to medically retire from the Marine Corps with a back injury in 1959. He worked as an inspector of standards for his home state of Massachusetts. In a 1970 Newsweek interview, he talked of his lifelong burden of guilt and how he prayed to God every day to keep the boys in his safekeeping and for forgiveness. Matthew McKeon died on November 11, 2003 at the age of 79.
John C. Stevens wrote a book about the Ribbon Creek incident called Court Martial At Parris Island. He tracked down and interviewed many of McKeon's recruits. Stevens pointed out that, with one exception, all of them spoke in favorable terms about their former drill instructor. They claimed he was not the sadist portrayed by the prosecution.
The incident led to several changes in Marine Corps recruit training, following on reforms that had begun in the early 1950s, in the wake of the Korean War and a large influx of recruits into the Corps. Recruit Training Commands (RTC), commanded by Brigadier Generals directly appointed by and answering to the Commandant, was established aboard both Parris Island and MCRD San Diego. Within these commands, officers were selected to oversee recruit training down to the Series level. New Drill Instructor schools were established within each command, and DIs were more carefully selected. The number of Drill Instructors assigned to each platoon was expanded to three, rather than two, and the role of the Drill Instructor was reformed to emphasize example, leadership, persuasion and psychology in the process of recruit training. Special Training Company (STC) was also established to provide remedial training to recruits needing additional physical conditioning, motivation, or education and rehabilitation to recruits suffering from medical conditions. The campaign cover was introduced as a distinctive element of Drill Instructor dress, in part to recognize a new norm of professionalism and specialization within the Drill Instructor billet, and in part to signify a break from the "old" era of recruit training and the "new".
William Baggarley McKean BG, USMC, the Commanding Officer of Weapons Battalion at the time wrote the definitive book on the incident. Ribbon Creek LOC#58-12776.