As a result of this service to the Whig Party, Cooper was appointed by President Zachary Taylor as an associate justice of the Minnesota Territorial Supreme Court on March 15, 1849.

[1] When the Mdewakanton uprising broke Governor Alexander Ramsey asked him to go to Gull Lake and ascertain Chippewa Chief Hole in the Day's position.

[2] When he arrived at Hole in the Day's village, he learned the Sioux had attacked the Chippewa at Otter tail lakes.

[4][5] Along with Chief Justice Aaron Goodrich, Cooper was frequently criticized by members of the legal profession as unfit and incompetent.

Joseph Cooper died approximately two months later, and it was said that the pistol wound had hastened his death.