Diman was born in Bristol, Rhode Island, on August 5, 1795.

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.

Although he did not win the majority of votes, he was selected as the governor of the state by the General Assembly.

He died on August 1, 1865, and was buried in Juniper Hill Cemetery.