While in the United States House of Representatives, he had prominently opposed the 1867 effort to impeach President Andrew Johnson.

In the last half of the nineteenth century, two unrelated Iowans named James Wilson achieved high office, necessitating an early form of disambiguation.

[2] After serving as a harnessmaker's apprentice, he studied law in Newark alongside future U.S. Supreme Court Justice William Burnham Woods.

At the convention called to choose the Republican nominee to succeed Curtis, "it was a foregone conclusion that James F. Wilson would be the unanimous choice.

"[6] In October 1861 Wilson was elected to fill the vacancy, easily defeating Democrat Jairus Edward Neal.

[2] Rep. Wilson's first action in Congress was introducing a resolution prohibiting fugitive slaves from being returned to the South and ordering the dismissal of any military officer that instructs troops to do such, which was enacted.

A business lawyer, Wilson advocated moderating the income tax among lower brackets and "reasonable" levels of protectionism.

Along with other Radical Republicans, Wilson adamantly supported a military occupation of the South during Reconstruction on the grounds that it was the only effective means of ensuring security.

Wilson warned the that a broad interpretation of impeachment powers, as Boutwell championed, in theory could allow the House to effectively dictate the policy of presidents.

[4][19] Before the vote, Wilson expressed an opinion representative of those expressed during debate by many Republicans that had previously voted against the impeachment resolution brought by the Judiciary Committee at the close of the 1867 impeachment inquiry against Johnson,[14] declaring that, The considerations which weighed upon my mind and molded my conduct in the case with which the Committee on the Judiciary of this House was charged are not to be found in the present case.

[14] Wilson opined that in the previous impeachment vote, Johnson had not committed any action that was a crime under either common law or statute.

Wilson declared that Johnson had mistakenly been emboldened after he was not impeached in December 1867 and had proceeded to commit an act that constituted clear impeachable conduct,[14] declaring, He mistook our judgment for cowardice, and worked on until he has presented to us, as a sequence, a high misdemeanor known to the law and defined by statute.

During the trial, Wilson sparred with Maine senator William P. Fessenden, who claimed that he broke from "his usual discretion.

Senator was to propose an unsuccessful constitutional amendment to more explicitly authorize the federal government to adopt laws that protect civil rights from violations by private citizens, to nullify the Supreme Court's ruling two months earlier in the Civil Rights Cases, 109 U.S. 3 (1883).

In 1890, Wilson was one of three Senators mentioned as potential nominees to fill the vacancy on the U.S. Supreme Court created by the death of Justice Samuel F. Miller of Iowa.

[27] President Benjamin Harrison instead picked Michigan judge Henry Billings Brown, who would later write the Supreme Court's opinion upholding "separate but equal" racial segregation in Plessy v. Ferguson, 163 U.S. 537 (1896).