Isaac Newton Arnold (November 30, 1815 – April 24, 1884) was an American attorney, politician, and biographer who made his career in Chicago.

After returning to Chicago in 1866, he practiced law and wrote biographies of Abraham Lincoln and Benedict Arnold.

When arguing before the Supreme Court, he twice was the opposing counsel of Illinois attorney Abraham Lincoln,[1] who he became acquainted with and befriended.

[5][6] A strong supporter of President Lincoln during his tenure in Congress, Arnold pushed emancipation in the territories and nation.

[2] In March 1862, during the American Civil War, Arnold introduced a bill to abolish slavery in U.S. territories, which became law in June 1862.

In 1864 Arnold faced a strong challenge from the Democrat John L. Scripps, the postmaster in Chicago, whose appointment he had opposed.

In addition, German Americans made up 25 percent of Arnold's constituents in 1860, and they were unhappy with him about continued drafts of men into the Army.

[2] After he lost his seat in congress, Arnold accepted a presidential appointment from Lincoln as the Sixth Auditor of the Treasury Department.

[2] He did years of research on an earlier historical figure, writing a biography entitled The Life of Benedict Arnold: His Patriotism and His Treason (1880).