After graduating in 1853, he became a school teacher in Morgantown, West Virginia and began to read law under his friend's father Edgar C. Wilson.

[1][2][3] Mitchell relocated to Winona, Minnesota with Eugene McLanahan Wilson shortly after passing the bar.

In 1877, Mitchell was called upon to serve on the Minnesota Supreme Court pro hac vice to hear a case where two of the sitting justices were involved as counsel.

[1][2][3] At one point, President Benjamin Harrison nominated Mitchell for an open seat on the Eighth Circuit Court of Appeals, but he eventually withdrew from consideration.

[citation needed] After it was incorporated into the U.S. as a protectorate, Mitchell was offered the position of Chief Justice of the Supreme Court of Puerto Rico but he declined.

In his eighteen years on the court, Mitchell wrote more than 1,500 opinions touching on all areas of Minnesota state law.

[1] In a letter to a friend in Minnesota, renowned Harvard Law Professor James Bradley Thayer wrote:[1]: 71 I have long recognized Judge Mitchell as one of the best judges in this country, and have come to know also the opinion held of him by lawyers competent to pass on an opinion on such a question ... Pray do not allow your state to lose the services of such a man.