Molenbeek, Union were promoted back to the Belgian First Division A, marking their first appearance in top-flight football in 48 years.

From 1933 to 1935 the team played 60 consecutive matches undefeated, setting a still unbeaten record in Belgium, winning three league titles in a row across that period led by captain Jules Pappaert.

[4][5] With its first national title barely a few years after its foundation, Union quickly became a superpower and supplier to the Belgian team for the Olympic Games in 1920.

Between 1958 and 1965, the club had a brief spell of European success, playing in the Inter-Cities Fairs Cup and reaching the semi-finals in the 1958–60 edition after a two-legged victory against A.S. Roma.

Promotion back to the First Division was a prospect, until a poor climax to their campaign post-Christmas, leading to the sacking of coach Tshupula Kande, replaced by Alex Czerniatynski.

In May 2007, long-standing clubchairman Enrico Bove resigned along with several other members of the board of directors, and the club suffered another relegation to the Third Division.

Bove returned as chairman in 2010 with the backing of new Italian sponsors, but Union's prospects on the pitch continued to suffer.

However, KVK Tienen were found guilty of making irregular payments and were automatically relegated to the Fourth Division, earning Union a reprieve via a play-off, which they won 1–0 against RFC de Liege.

[6] On 21 May 2018, Tony Bloom, chairman of English Premier League side, Brighton & Hove Albion was confirmed as the majority shareholder.

A change of coach in May 2018 saw Marc Grosjean replaced after three years of finishing 6th, 4th and 6th by Luka Elsner, with Union contesting at the top of Division 1B in 2018–19, which that season was divided into South American-style "opening" and "closing" competitions, framed in the "apertura and clausura" style seen in many countries in the Americas.

Placed in a group of six with every other opponent being from the top division, USG went unbeaten at home, beating Kortrijk, Waasland-Beveren and Cercle Brugge, drawing with Zulte Waregem and Excel Mouscron.

With Elsner taking up a coaching job in France with Amiens SC, Thomas Christiansen was brought in as his replacement, with Union again falling marginally short in both the Opening and Closing Tournaments.

The summer of 2020 saw Felice Mazzu appointed as first-team coach, assisted by Karel Geraerts, with a number of interesting young talents signed from lower-division football both in Belgium and abroad, such as Dante Vanzeir, Deniz Undav, Christian Burgess, Teddy Teuma, and from Virton following that club's denial of a First Division B licence, goalkeeper Anthony Moris and Loïc Lapoussin.

[10] Union in their next match a week later hosted reigning champions Club Brugge on 1 August 2021 in their first home game in the top division since 1973, being edged out 1–0 through a late Eduard Sobol winner.

Union went top of the Pro League on 17 October 2021, passing previous leaders Eupen by virtue of a 4–1 win over Seraing, and would remain there until May.

Due to the novelty of being crowned unofficial winter champions, Union were awarded a special trophy by the newspaper HLN.

[11] In a difficult January run against all of the previous season's top four, Union beat Anderlecht, Genk and Royal Antwerp, drawing 0–0 away to Club Brugge.

Union started the play-offs strongly, beating Anderlecht 3–1 and thus becoming the first club ever to inflict three defeats on the Mauves in the same league season.

The title race swung in the direction of Bruges in back-to-back games between Union and Club, with the champions earning a tight 2–0 win in Brussels to draw level on points,[12] with Club going top under the "half-points" rule, as Union had needed their tally from the regular season rounded-up.

Club Brugge won the midweek return 1–0 at the Jan Breydel through a rebounded own goal from goalkeeper Anthony Moris to take a three-point lead with two games to go.

[13][14] Club Brugge clinched the league crown in their next (and penultimate) game against Royal Antwerp,[15][16] despite Union beating Anderlecht for the fourth time in the season 2–0 at Lotto Park.

Despite an early exit from the UEFA Champions League at the hands of Rangers after a 2–0 "home" win at OH Leuven's Den Dreef stadium[23] and a 3–0 loss at Ibrox,[24] Union dropped into the UEFA Europa League and won their group, earning four victories from their six games against Union Berlin, Braga and Malmö.

They met for the first time since 1979 in the Belgian Cup in 2018, with Union stunning Anderlecht 3–0 at the Constant Vanden Stock Stadium.

At the end of the season, however, Union coach Mazzu departed for cross-town rivals Anderlecht and was replaced by his assistant Karel Geraerts.