The 1920–21 season was the 20th year of competitive football played by Cardiff City F.C. and the team's first in the Football League. In a ballot by members of their new league, Cardiff were voted into the Second Division and won their first match 5–2 against Stockport County. Cardiff finished the season tied on points with first-placed Birmingham, with 58 of a possible 84 points. The winner was therefore decided via goal average, with Cardiff placing second by a margin of 0.235. The two sides were both promoted to the First Division.