Mitchell was elected to the House of Representatives in 1934, defeating African American congressman Oscar De Priest, who was a Republican.

During the election campaign, Mitchell emphasized his support for the New Deal and President Franklin D. Roosevelt's public relief programs,[3] in addition to criticizing De Priest's opposition to segregation as ineffective.

He filed a lawsuit against the Illinois Central and Rock Island Railroads after he was forced into a segregated train car just before it passed into Arkansas.

As his last congressional act, Mitchell condemned politicians as preferring the Axis powers over giving Negros any rights, comparing the atrocities of the Nazis and Japanese with lynchings such as those that had recently occurred in Shubuta, Mississippi.

[1] In one instance, the National Association for the Advancement of Colored People deemed his introduced anti-lynching bill as too lenient.