English: Gravestone of John M. Lloyd in section one of Mount Olivet Cemetery in Washington, D.C., in the United States.

Lloyd was born probably in Prince George's County, Maryland, in either 1835 or 1836. A relatively uneducated bricklayer, his father moved the family into the District of Columbia in 1850 when John was 14. The city reorganized its police force (previously active only at night) in 1851, and John became one of the city's first policemen a short time later (just when is not clear). The American Civil War broke out in April 1861. Lloyd was a Confederate sympathizer, and left the police department in 1862. He moved back to Prince George's County, where he worked as a bricklayer and tavern keeper.

On December 1, 1864, Lloyd rented Mary Surratt's tavern in Surrattsville (now Clinton), Maryland. He quickly got caught up in the conspiracy by John Wilkes Booth, John Surratt, David Herold, Lewis Powell, and George Atzerodt to kill Abraham Lincoln. Interrogated after the crime, Lloyd confessed his role in the conspiracy, and investigators decided to use his testimony to convict the main conspirators. Lloyd testified at the trial of Mary Surratt, Powell, Atzerodt, and Herold in May 1865. His testimony, which was heavily corroborated, likely convicted Mary Surratt. The four were hanged on July 7, 1865, and Lloyd was never charged with any crime. (The law at the time did not permit it, if he testified against his co-conspirators.)

Lloyd moved out of Surrattsville in October 1865, and back to D.C. He worked as a bricklayer for the rest of his life. He died on December 18, 1892, after scaffolding he was standing on collapsed. He is buried about 150 feet from Mary Surratt.