James Grimes was born in Deering, New Hampshire, on 20th October, 1816. After graduating from Dartmouth College he studied law and worked as a lawyer in Wisconsin Territory.
Grimes served in the Iowa House of Representatives in 1838-1839 and 1843-44. He served as Whig Party governor of Iowa (1854-1858) before joining the Republican Party. Grimes was elected as a member to the Senate in November 1858.
In July, 1861, Grimes was a member of a group of politicians, including Benjamin Wade, Lyman Trumbull and Zachariah Chandler, who witnessed the Battle of Bull Run. The battle was a disaster for the Union forces and at one stage Grimes came close to being captured by the Confederate Army. After arriving back in Washington, Grimes was one of those who led the attack on the incompetence of the leadership of the Union Army.