New signing Jimmy Gill was the club's top goalscorer with 20 goals in all competitions. Cardiff fielded 29 players in 1920–21, Billy Hardy playing in the most games. In the Welsh Cup, Cardiff, the holders, were beaten by Pontypridd after a fixture clash forced them to field a reserve side.
They were knocked out by Second Division Wolverhampton Wanderers. Cardiff reached the semi-final of the FA Cup, defeating First Division sides Sunderland and Chelsea. They finished the season tied on points with Birmingham, who were champions on goal average both teams were promoted to the First Division. Cardiff City F.C.'s 1920–21 season was the 20th season of competitive football for the club and their first in the Football League, having been voted into the Second Division.
