Rotherham United FC was founded in 1925 after a merger between Rotherham County and Rotherham Town. However, the club's roots go back to 1870 when the club was formed in 1877 as  Thornhill Football Club (later Thornhill United). For many years the leading team in the area was Rotherham Town, who spent three seasons in the Football League while Thornhill United were still playing in the Sheffield & Hallamshire League. By the turn of the century, however, Rotherham Town had resigned from the Football League and briefly gone out of business before joining the Midland League. Meanwhile, Thornhill's fortunes were on the rise to the extent that in 1905 they laid claim to being the pre-eminent club in the town and changed their name to Rotherham County. For a period both clubs competed in the Midland League, finishing first and second in 1911-12. When the Great War ended, it was decided to extend The Football League by creating four new places in Division Two. Rotherham County, who had been champions of the Midland League in 1915 before the competition was suspended, were elected to one of the vacancies. 

The club have spent the majority of their history in the Football League's third tier, though their most recent success came in the early 2000s when they found themselves competing in the Football League Championship, the second tier of English football. Since that time, Rotherham has struggled with finances, entering into administration twice in as many years. Rotherham United were therefore one of three teams to start the 2008/09 League Two season on negative points, being deducted 17 points by the Football League. They  quickly wiped out this deficit, however, and were in contention for a play-off place until very late in the season.  Rotherham gained promotion to League One by finishing second in League Two during the 2012-13 and were promoted again the following season  through the League One play-offs beating Leyton Orient via a penalthy shoot-out at Wembley.  The Championship proved a greater challenge and relegation was only avoided with a 2-1 home victory against Reading in the penultimate game of the season. Rotherham finished the 2016-17 season bottom of the league and were relegated to League One, but bounced back at the first attempt, after defeating Shrewsbury in the Play-off final. The club bounced between the Championship and League One for the next four years returning to the second tier in 2022 and winning the EFL Trophy at Wembley. However, they were relegated at the end of the 2022/23 season.  Rotherham played all their home games at Millmoor in Rotherham from 1907 to 2008 before moving to the 12,000 all-seated New York Stadium in 2012.