He attended common schools and then studied law at Trinity College in Hartford, Connecticut, graduating in 1832.

During his 18-year term, he served as Chairman of the Committee on Ways and Means (Thirty-Fifth Congress) and came to be regarded as a champion of government bounties to soldiers, aid to railroads, and inexpensive postage.

At the outbreak of the Civil War in 1861, Phelps returned to Springfield and enlisted as a private in Captain Coleman's Company of Missouri Infantry (Union).

In March 1862, Phelps led his regiment in the fierce fighting at Pea Ridge in Arkansas.

In July 1862, he was appointed by President Lincoln as Military Governor of Arkansas, but he resigned the position due to ill health.