Participation in the My Lai Massacre: Mitchell participated in the mass killing of unarmed Vietnamese civilians at My Lai on March 16, 1968. Eyewitnesses testified he fired into groups of villagers at the drainage ditch where dozens to hundreds of civilians were executed.

Court-Martial and Acquittal: Mitchell was tried by court-martial for assault with intent to murder approximately 30 Vietnamese civilians at the ditch. Despite testimony from multiple eyewitnesses placing him as an active shooter, the military jury acquitted him in November 1970.

Firing at the Drainage Ditch: Multiple witnesses testified that Mitchell was among the soldiers who fired M16 rifles into groups of unarmed civilians herded into a drainage ditch on the eastern edge of My Lai. The ditch killing was one of the most concentrated acts of mass murder during the operation.

Claims of Following Orders Defense: Mitchell's defense centered on the claim that soldiers were following orders from superior officers, particularly Captain Ernest Medina and Lieutenant William Calley. This defense proved effective with the military jury despite the principle that orders to commit war crimes are unlawful.

Peers Commission Testimony: Mitchell was identified in the Peers Commission investigation as one of the primary participants in the massacre. The commission, led by Lieutenant General William Peers, recommended his prosecution along with dozens of other soldiers and officers.

Witness Testimony Against Him: Several fellow soldiers testified that they saw Mitchell firing at civilians. Vietnamese survivors who were wounded but survived the ditch killings also provided testimony identifying American soldiers at the scene.