John was born on the 2nd of July, 1872, in Worcester County, Maryland and joined the US Navy. He was deployed to China in support of the Boxer Rebellion and it was his actions as a Gunner's Mate Second Class that would earn him the Medal of Honor. The citation reads:

In action with the relief expedition of the Allied Forces in China, 13, 20, 21 and 22 June 1900. During this period and in the presence of the enemy, Chatham distinguished himself by his conduct.

John Purnell Chatham died on the 3rd of October, 1914 at the age of 42 and is buried with his wife Serophia in Parsons Cemetery in Salisbury, Maryland.