Diman was born in Bristol, Rhode Island on August 5, 1795. He worked in a counting-house for over two decades. He was then engaged in the whaling and mill businesses. He served in the Rhode Island Militia and later became Brigadier General. He became a member of the Rhode Island House of Representatives for many terms.
He became Lieutenant governor of Rhode Island for three years before winning election as the governor of Rhode Island. He was a Law and Order party candidate. Although he didn't win the majority of votes, he was selected as the governor of the state by the General Assembly. He held the governor's office from May 6, 1846 to May 4, 1847.
- Sobel, Robert and John Raimo. Biographical Directory of the Governors of the United States, 1789-1978. Greenwood Press, 1988. ISBN 0-313-28093-2