Thomas Preston served as a captain in the 29th Regiment of Foot, part of the British garrison sent to Boston during the mounting dispute over imperial taxation and military occupation. On March 5, 1770, he led soldiers to support a sentry outside the Custom House as an angry crowd gathered. The confrontation ended with troops firing into the civilians, killing or mortally wounding five people. Whether Preston had ordered the firing became the central issue in the criminal case against him.

He was arrested and held while authorities postponed proceedings to reduce public tension. Robert Auchmuty, John Adams and Josiah Quincy undertook his defense. Preston was tried separately from the enlisted men in October 1770. The prosecution argued that an order to fire would make him responsible for the deaths; the defense challenged the witnesses' ability to identify such an order amid noise, confusion and conflicting calls.

Testimony placed Preston in front of his men when the shots began, and witnesses disagreed over other important details. The jury acquitted him. The soldiers' subsequent trial produced six acquittals and two manslaughter convictions, rather than a finding that all had acted on a proven command from their captain. Preston left Boston after the proceedings and returned to Britain. His later life is less clearly documented than this episode of service. The trial records examine military responsibility, despite simplified contemporary images and partisan accounts.