Willis played for nine seasons in the majors as a relief pitcher for the Detroit Tigers, Cincinnati Reds, Chicago White Sox and Minnesota Twins (with whom he won the 1991 World Series).

[2] A year later, Willis was selected by the Detroit Tigers in the 23rd round (581st overall) of the 1983 Major League Baseball draft, and he signed with the team.

[3] Willis made his major league debut with the Tigers on June 9, 1984 against the Baltimore Orioles, tossing 2+1⁄3 scoreless innings in a 4–0 loss.

[5] On September 1, Willis was acquired by the Cincinnati Reds to complete an earlier trade for relief pitcher Bill Scherrer.

[14] In 2015, Willis was named to be the pitching coach for the Columbus Clippers, marking a return to the Cleveland Indians organization.

[17] Willis has had five pitchers win Cy Young Awards during his time as their pitching coach: CC Sabathia in 2007, Cliff Lee in 2008, Félix Hernández in 2010, Rick Porcello in 2016, and Shane Bieber in 2020.

[1][18] In May 2022, Willis assumed managerial duties for a few games after a COVID-19 outbreak within the team sidelined multiple coaches, including manager Terry Francona.