A native of Warwick, Rhode Island, Mills joined the Alpha Delta Phi fraternity and graduated Cum Laude from Bowdoin in 1972 and then went on to pursue graduate degrees at Syracuse University in 1976 and the Columbia University School of Law in 1979, where he was a Harlan Fiske Stone Scholar. Upon graduating, he soon began working at the law firm, Debevoise & Plimpton, where he became a partner in 1986. On December 19, 2008, his wife, Karen Gordon Mills, was nominated by President Barack Obama to serve in his administration as Administrator of the Small Business Administration, in which role she served in until February 11, 2013.
A member of the Board of Trustees from 1994 through 2000, Mills became president of Bowdoin in October 2001. Since then, Mills has dramatically changed Bowdoin's curriculum and campus. As part of a master plan first designed by Skidmore, Owings & Merrill in 2004, the college has built new residential dorms, a recital hall, a hockey arena, a fitness center, converted one of the college's pools into an architecturally distinctive recital hall, and has undergone a highly publicized renovation to the Bowdoin College Museum of Art.
In 2011, Bowdoin set a record low rate of admissions for the class of 2015 at 15.7%. Three years earlier, in 2008, it was recognized as "School of the Year" by College Prowler. Additionally, that January, Mills announced that all student loans would be replaced by grants beginning in September.
Mills presented the Bowdoin Campaign in 2006, a $250 million fund-raising campaign set to be finished in June 2009 and focusing on new faculty positions and financial aid. Aided by a $10 million gift by Subway Sandwiches co-founder Peter Buck, the goal was met that February. In response to the global financial crisis, in September 2008, Mills announced that the college would slow down the rate of new capital projects and faculty positions but would retain job security at the college.
In April 2014, Mills announced he would "step down as president of the College ... at the conclusion of the 2014-15 academic year." He officially stepped down on July 1, 2015, and was succeeded by Clayton Rose.
In March 2017, Mills was appointed deputy chancellor and chief operating officer at the University of Massachusetts Boston. In this role, he will oversee the academic and research program and campus operations.