Mills' no-no is the 16th in Cubs history. He becomes the 12th hurler to accomplish the feat for Chicago and the first since righty Jake Arrieta did it in April 2016.
The 28-year-old admitted he went to the clubhouse between the eighth and ninth innings to try to calm himself down, according to Jordan Bastian of MLB.com.
Shortstop Javier Baez recorded the final out of the game on a routine grounder.
"Javy caught the ground ball and screamed before he threw it, so that was kind of cool. Talk about confidence," Mills said postgame, according to Jesse Rogers of ESPN.
Mills threw 114 pitches in his 15th career start. He registered five strikeouts and walked three batters.
Chicago White Sox ace Lucas Giolito was the only other pitcher to throw a no-hitter this season prior to Mills' performance.
This marks the first time in MLB history that both Chicago teams have completed no-hitters in the same season.