He spent 12 seasons in Major League Baseball (MLB) with the New York Yankees (1990–1991), Baltimore Orioles (1992–1998, 2000–2001) and Los Angeles Dodgers (1999–2000).

He attended Tuskegee University, but transferred to Polk Community College after one year when the former dropped baseball scholarships.

Mills converted to a relief pitcher during his three seasons in the Yankees' farm system, going 12–24 with a 4.25 ERA and thirteen saves.

Numbers that high are typically only worn by players in spring training (when teams have considerably larger rosters than they do during the regular season).

Mills chose to wear it as a motivating factor, to remind himself that his job was not necessarily any more secure than that of someone in spring training should he perform poorly.

[7] Mills attempted comebacks with the Montreal Expos in Spring training 2002 and the Tampa Bay Devil Rays in 2004, but was essentially away from the game for five years before signing a minor league deal with the Detroit Tigers in 2007, and spending one season with the Erie SeaWolves.

[8] Mills returned to his alma mater Kathleen High School as a physical education teacher and head coach of its varsity baseball team from 2009 through 2011.

His return to the Orioles organization in 2012 began a five-year span as pitching coach with the Aberdeen IronBirds (2012–2013),[9] Delmarva Shorebirds (2014) and Bowie Baysox (2015–2016).

[11] Despite the Gulf Coast League cancelling its playoffs due to the threat of Hurricane Dorian, Mills was named Manager of the Year after guiding the Orioles to a league-best 38–15 record in his first full season at the helm.