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Born in Norton, Massachusetts, Leonard was a member of a prominent family who made their fortune from their iron works in Taunton, Massachusetts. After graduating from Harvard College, Leonard began to practice law in Taunton.
During the Revolutionary crisis, the British Parliament passed the Massachusetts Government Act, which, among other things, abolished elections for the Massachusetts Governor's Council and instead called for the councilors to be appointed by the royal governor. Leonard accepted an appointment by Governor Thomas Hutchinson to this new royal-controlled Council. Massachusetts Patriots were outraged, and attacked Leonard's house. He fled to British-occupied Boston for safety.
In 1774 and 1775, Leonard, writing under the name "Massachusettensis," wrote a series of letters in support of royal government that were published in a Loyalist Boston newspaper, the Massachusetts Gazette. John Adams, writing as "Novanglus," answered the letters in the Boston Gazette. The exchange ceased with the Battles of Lexington and Concord. Many, including Adams, erroneously believed that Jonathan Sewall had written the Massachusettensis letters.
Leonard's 1829 death in London was possibly a suicide.
- Joseph F. Clarke (1977). Pseudonyms. BCA. p. 112.