 In 1859 he moved to Ann Arbor, Michigan and became one of the University of Michigan Law School's first professors.He would go on to play a major role in the development of the University and the Law School, serving on faculty until 1884, including a long stint as the law school's dean from 1871 until 1883.

in 1864, Cooley was elected to the Supreme Court of Michigan, and served as the chief justice for 20 years.Politically, he remained a Republican, and even considered running for Congress in 1872.