The original Trewern United club was founded in 1954. Trewern United were
Montgomeryshire League champions in 1961–62. In 1964, they were accused of "throwing" a match, after a 10–1 loss to Newtown Amateurs on 14 March. The club dismissed the accusation as "ridiculous". In February 1965, five Trewern players were warned by the
Football Association of Wales for leaving the field without permission, in a game away at
Carno on 12 December 1964. In the 1992–93 season, they were runners-up in the Montgomeryshire League Division Two, and were promoted to Division One. A 4–1 loss to Mochdre late in the season confirmed it was Mochdre who were champions. The club remained active in the Montgomeryshire League until folding in 2005. For the 2007–08 season Trewern United reformed and were again members of the Montgomeryshire League. This club folded in 2010. In January 2016 it was announced that Trewern Football Club would reform and rejoin the Montgomeryshire League. The club had secured the use of the pitch and changing facilities at Buttington Trewern School. This was confirmed at the league's
annual general meeting in June, with the club using the name Trewern United, like the village's the previous clubs. Their first match was in July, in a friendly against a team of players from the previous Trewern United. It finished 3–3 and the team of former players won after a penalty shootout. The club competed in the
Welsh Cup in both of its first two seasons (
2016–17 and
2017–18), but lost in the first qualifying round both times. In May 2019, Trewern won 4–0 against
Abermule reserves to reach the final of the Consolation Cup. Later that month, they played
Kerry reserves in the final, and a 115th minute winner from Matt Windsor secured Trewern's first silverware since reforming. In the 2021–22 season, they were winners of the Boys and Boden Montgomeryshire League Cup, and runners-up in the Llansantffraid Village Cup. After the season they joined the
Central Wales Football League's newly formed Northern Division. This was their first time ever playing in the Central Wales League. In 2025, Trewern were runners-up in the
J. Emrys Morgan Cup, losing to
Barmouth & Dyffryn United after a penalty shoot-out. In December 2025, the club withdrew from the Central Wales League, with plans to reform in the Montgomeryshire League for the 2026–27 season. ==Ground==