Due to an injury to Trevor Francis he started in the 1–0 victory over Hamburger SV in the 1980 European Cup Final, staged at the Santiago Bernabéu Stadium, and being aged 18 meant he became the youngest player to appear in a European Cup final. Mills played more frequently in 1979–80, making 20 appearances and scoring 1 goal, as Forest finished fifth in the league. He played in the 2–1 home win over Valencia in the first leg of the 1980 European Super Cup, but was not selected for the second leg which the Spanish club won 1–0; this meant Valencia became the first team to win the competition on the away goals rule. Mills missed only one league match for Forest from February 1981 to the end of 1980–81, and finished the season with 34 appearances and 7 goals, with Forest placed seventh in the First Division. He appeared more sporadically in 1981–82, making 17 appearances and scoring 1 goal, not playing more than seven league matches in succession.