In 1968, the club joined the United Counties League, and immediately earned promotion after finishing as runners-up.

After starting their UCL life at Dallington Park, the club moved to Duston High School for two years before arriving at its current home at Kingsthorpe Mill in 1971.

Fortunes were transformed by the arrival of ex-Northampton Town boss John Petts as manager and he led the Millers back to the Premier Division in 1985.

Since then, Spencer has been one of the stronger sides in the league, rarely finishing out of the top ten, and winning the championship in 1991–92 with 101 points.

This was followed by 2–0 victory over local rivals ON Chenecks on 6 April sealing the UCL Division 1 title with 3 games to spare.