After working in a New York shipping office, he was a clipper-ship officer and then engaged in business in Chicago. In 1860 he moved to Camden, New Jersey, which was thereafter his home.
Commissioned captain in the 5th New Jersey Infantry, August 28, 1861, he became lieutenant-colonel of his regiment after the Seven Days and colonel after Second Bull Run. On the morning of May 3, 1863, at Chancellorsville, he led his regiment against three heavy Confederate attacks, and then led his brigade in counter attacks, capturing eight colors and a thousand prisoners, but finally having to retire when ammunition gave out. Sewell was wounded there and at Gettysburg and was invalided after Spotsylvania, but returned to duty shortly as colonel of the 38th New Jersey Infantry and served until June 30, 1865. He was brevetted brigadier-general of volunteers for his gallantry at Chancellorsville.
For the rest of his life, he was prominently associated with the railroads of southern New Jersey, first with the Camden & Amboy Railroad and particularly with its subsidiary the West Jersey, which grew from thirty-seven miles in 1867 to 309 miles in 1896. After 1870, it was an autonomous part of the Pennsylvania system, controlling several subsidiaries of its own. Sewell was its general superintendent until 1881, when he succeeded A. J. Cassatt as vice-president, becoming a director in 1885. In 1899 he was made president of the West Jersey & Seashore Railroad, formed in 1896 by consolidating the West Jersey and its subsidiaries.
As state senator from Camden County from 1872 to 1881 and president of the Senate in 1876, 1879, and 1880. In 1873 he was a member of Governor Parker's staff with the rank of major-general, and that same year he secured a charter for the Camden Safe Deposit & Trust Company, of which he was an incorporator and thereafter a director.
In 1881 he was elected to the United States Senate in place of Theodore Fitz Randolph. He sought reelection in 1887, opposing Gov. Leon Abbett, but after weeks of deadlock the choice fell on a dark horse, Rufus Blodgett. Sewell returned to the Senate in 1895, however, and was a senator until his death.
He enjoyed considerable influence in Washington, particularly with President Harrison. During these years he retained his interest in military affairs. He commanded a brigade of the National Guard and ably handled the situation at Phillipsburg, New Jersey, in the railroad strike of 1877. In 1898 he was appointed major-general of volunteers, but did not see active service.
He died at his home in Camden.
He had the faculty of summing up a complicated situation in a few words; he seldom made orations, and was doubtless at his best in the informal gatherings where the real decisions were made.
Sewell was married twice; his first wife died in 1861 and at the close of the war he married Helen L. Heyl, who with five children survived him. Two of his sons became army officers.