[2] On 1 December 2017, Campbelltown City Council informed the Football Federation Australia (FFA) of their plans to be part of an expanded A-League by gaining entry into the competition in two years.

[4] The proposal by the council soon gained support from other committees in the ensuing months, namely Camden and Wollondilly Shire, and from the Macarthur Football Association, which represented dozens of community clubs and over 10,000 registered players in the region.

[7] The bid was dubbed "United for Macarthur" and launched that month to allow local businesses and people to support a community-based and owned team proposal.

[8][9] The "United for Macarthur" campaign gained further support from A-League club Sydney FC and its chairman, Scott Barlow, who also issued criticism towards the Southern Expansion (Sutherland-St George Illawarra regions) proposal that he stated was "deeply flawed".

[15] Prior to the final decision, the Southern Expansion bid attempted a merger with Macarthur which included $20 million in the offer that was rejected by Gino Marra.

[16] The FFA announced on 13 December 2018 that Macarthur South West Sydney alongside Western Melbourne were confirmed to be the two new expansion clubs in the next two A-League seasons.

[22][23][24] Macarthur also announced their home ground to be Campbelltown Sports Stadium and its training headquarters to be based at a new Centre of Excellence, which was under construction at the time.

[22] On the same day as their unveiling, Ante Milicic was announced as the inaugural head coach of Macarthur FC and was set to start in the club's first season.

[57] Cypriot-Australian footballer Antonis Martis was also announced, just two days later, coming in on a season-long loan from Danish club FC Midtjylland.

[63][64][65] Antony Golec,[66] Lachlan Rose,[67] and James Meredith were among the last signings made by the club before the opening match of the 2020–21 A-League season.

The Bulls went on to win 1–0 over the Wanderers, with Mark Milligan scoring the first league goal for the club in the 72nd minute of the match, assisted by Beñat from a free kick.

[78] From the end of March to early April, Macarthur achieved three consecutive victories, triumphing over Western United, Wellington Phoenix and Perth Glory, leading them to take second place behind the Mariners.

The club went down to 10 men after James Meredith was sent off in the 75th minute before a goal each in extra time by Charles M'Mombwa and Michael Ruhs sealed the Bulls qualification to the semi-final.

[84] The venue was set at Jubilee Stadium, instead of City's home ground in Melbourne, due to crowd restrictions imposed as a result of the COVID-19 pandemic in Victoria.

[88] Four days prior, Macarthur FC confirmed the departure of Ivan Franjic, Loic Puyo, Walter Scott, Yianni Nicolaou, Milislav Popovic and Kyle Cimenti from their squad.

The two clubs first met in the opening round of the 2020–21 A-League season on 30 December 2020, with Macarthur winning the match 1–0 after a goal scored by Mark Milligan.

Macarthur FC vs. Western Sydney Wanderers Note: Flags indicate national team as defined under FIFA eligibility rules; some limited exceptions apply.