Mitchell was elected as a Democrat to the 61st United States Congress to fill the vacancy caused by the death of Charles Q. Tirrell, serving from November 8, 1910, to March 3, 1911.

However, he lost a simultaneous election to the 62nd United States Congress, and therefore only served until the completion of the open term.

He was elected to the Sixty-third Congress to fill the vacancy caused by the resignation of John W. Weeks and served from April 15, 1913 to March 3, 1915.

On February 5, 1915, Woodrow Wilson nominated Mitchell for the position of United States Marshal for Massachusetts.

His nomination was confirmed by the United States Senate on March 9, 1920 and he took office on April 1, 1920.