Born in Providence, Arnold received his early education under private tutors, then graduated from Brown University in 1841 and Harvard Law School in 1845.

In 1859, he was elected an Associate Fellow of the American Academy of Arts and Sciences,[1] where he wrote the two-volume History of the State of Rhode Island and Providence Plantations in 1859.

Shortly after the outbreak of the Civil War, Arnold was appointed as a military aide to Governor William Sprague with rank of colonel, and he raised the 1st Rhode Island Battery of light artillery, which went to Washington, D.C., and was mustered into the Union Army for three months.

[2] He was elected as a Republican to the U.S. Senate to fill the vacancy caused by the resignation of James F. Simmons, and he served from December 1, 1862, to March 3, 1863.

He wrote, "I have brought up my cousin for years to make her my wife, for I am so fastidious and particular on that matter that I knew I never should find a lady to suit me in all respects unless I educated her for the purpose.