Mills' first formal teaching assignment was as a lecturer at the Faculty of Law at the University of Ghana. He spent close to twenty five years teaching at Legon and other institutions of higher learning. In 1971, he was selected for the Fulbright Scholar programme at Stanford Law School in the U.S. He returned to Ghana after receiving his PhD, to work at his alma mater, the University of Ghana, for 25 years. He became a visiting professor of Temple Law School (Philadelphia , USA), with two stints from 1978 to 1979, and 1986 to 1987, and was a visiting professor at Leiden University (Holland) from 1985 to 1986. During this period, he authored several publications relating to taxation during the 1970s and 1980s. Outside of his academic pursuits, Professor Mills was the Acting Commissioner of Ghana's Internal Revenue Service from 1986 to 1993 under President Jerry John Rawlings ,and the substantive Commissioner from 1993 to 1996. By 1992, he had become an Associate Professor of Law at the University of Ghana.