Born in Castlebar, Ireland, Sewell immigrated to the United States in 1851. He worked in the merchant industry in Chicago, Illinois before moving to Camden, New Jersey in 1860. During the Civil War, he served with the 5th New Jersey Infantry, beginning as a captain in 1861, and rising in rank to major general in 1865. He was awarded the Medal of Honor in 1896, for having assumed command of a brigade at the Battle of Chancellorsville, Virginia, May 3, 1863. The citation read:

"assuming command of the brigade, he rallied around his colors a mass of men from other regiments and fought these troops with great brilliancy through several hours of desperate conflict, remaining in command though wounded and inspiring them by his presence and gallantry of his personal example." After the war Sewell worked in the railroad industry in New Jersey before being elected to the state senate, serving from 1872 until 1881, and as the senate's president in 1876, 1879-1880.

Subsequently, he was elected to the United States Senate as a Republican and served from March 4, 1881, to March 3, 1887. During this period he chaired:

the Committee on Enrolled Bills (Forty-seventh Congress and Forty-eighth Congress), the Committee on Military Affairs (Forty-ninth Congress), and the Committee on the Library (Forty-ninth Congress). He also served as of the national commissioners for New Jersey to the World's Columbian Exposition in Chicago in 1893, was in command of the Second Brigade of the National Guard of New Jersey, and was appointed a member of the Board of Managers of the National Home for Disabled Volunteer Soldiers.

Sewell was a unsuccessful candidate for reelection in 1887, 1889 and 1893 but was again elected to the United States Senate in 1895 and served from March 4, 1895, until his death 27 December 1901 in Camden, New Jersey. During this term he chaired the Committee on Enrolled Bills (Fifty-fourth Congress through Fifty-seventh Congress).

Sewell was intermed in Harleigh Cemetery.