In the Welsh Cup, Cardiff were the holders entering the competition but were eliminated in the third round by Pontypridd after a fixture clash with a league match against Bristol City forced them to field a reserve side.

[6] Cardiff's election meant that they were placed in a higher division than the three sides that had finished above them, Portsmouth, Watford and Crystal Palace.

[8] Other new signings included Herbie Evans from local side Cardiff Corinthians,[9] goalkeeper Ben Davies from Middlesbrough, Tom Sayles from Sheffield, Ernie Gault and Jack Page from Everton and Laurence Abrams from Chelsea.

[10][11] Stewart also allowed defenders Patrick Cassidy and Kidder Harvey to leave the club, surprising many fans and local journalists given the two players' experience.

The pair, alongside Billy Hardy, had formed part of the "holy three", as they were known by fans in the Southern League; however, the emergence of Fred Keenor hastened their departure.

[12] The club announced a significant rise in ticket prices ahead of the new season, citing the increase in travelling and rail fares that would be required in the new division.

[13] At the annual general meeting on 16 August, plans for £20,000 worth of improvements to be made to the club's home ground, Ninian Park, to cope with spectator demand were authorised.

[16] Cardiff held their only preseason fixture on 18 August, with the first team squad being split into two sides to face each other at Ninian Park, where they played out a 2–2 draw.

Considered underdogs against the more experienced side,[18] Cardiff secured a 5–2 victory with two goals from new signing Gill and one each from Jack Evans, Billy Grimshaw and Keenor.

[19] The team's second match was the first Football League tie held at Ninian Park as 25,000 fans witnessed a goalless draw with Clapton Orient.

[10] At the time, the Football League used Charles Sutcliffe's fixture system that paired club's home and away matches against the same side on consecutive weekends or in close proximity wherever possible.

Cardiff remained unbeaten throughout the rest of October, recording two 3–0 victories over Fulham, home and away, and a win and a draw against Notts County.

[10] After a 0–0 draw at home against Blackpool, Blair returned to face his former club Wednesday and helped his new side record consecutive 1–0 victories, with both matches being decided by goals from George West.

[29] Cardiff endured a poor start to 1921, losing 1–0 to Bury on New Year's Day and then drawing 0–0 with promotion rivals Bristol City on 15 January.

[32] A 2–0 win over Barnsley, with goals from Gill and debutant Harry Nash, and a goalless draw with Stoke were not enough to stop the side slipping to fourth place soon after.

[10] The defeats resulted in Cardiff falling five points behind leaders Birmingham, although their prolonged FA Cup run had left them with three games in hand.

[36] Two days after losing to Port Vale, Pagnam helped the side return to winning ways by scoring the only goal in a victory over Leeds United on 28 March, although the match was marred by Barnett suffering a broken leg that ended his season.

[nb 2][38] Cardiff's final two matches of the season were against Wolverhampton Wanderers, the side that had eliminated them from the semi-finals of the FA Cup two months earlier.

In the first fixture, held at Ninian Park, 40,000 spectators witnessed Cardiff seal promotion with a 2–0 win,[39] with Gill and Pagnam the scorers.

Cardiff's promotion had already been guaranteed by a draw between The Wednesday and Bristol City earlier in the day and the club's directors let in much of the capacity crowd for free in celebration.

[10] The team were drawn against First Division side Chelsea, who would be playing their eighth match in the competition, having been taken to two replays by both Reading and Plymouth Argyle in the first and third rounds respectively.

[49] Chelsea's difficulty in overcoming opponents in the previous rounds meant that the match was seen as a closer contest than the divisional gap suggested.

[50] Cardiff's victory over Chelsea resulted in the side becoming the first Welsh team ever to reach the semi-final stage in the competition's history.

As semi-final ties are traditionally held at a neutral venue, the match was played at Anfield, the home ground of Liverpool, on 19 March.

Cardiff entered the competition as reigning holders having won the trophy for the second time in their history in April 1920 after defeating Wrexham 2–0.

The Welsh Cup fixture clash provided Len Hopkins, Tom Sayles and Tommy Wilmott with their only appearances of the season.

[56][57] FW = Forward, HB = Halfback, GK = Goalkeeper, DF = Defender Sources:[10][55] By winning promotion, Cardiff became the first Welsh side to reach the top tier of the Football League.

[59] A major increase in attendances following the move into the Football League and a long cup run significantly benefitted the club financially with the accounts being described as in "rude health" by the board.