Once clubs voted to curtail the season, it ensured the Millers would finish second in League One and be promoted alongside champions Coventry City.
Tranmere Rovers, Southend United and Bolton Wanderers are relegated to League Two.
Wycombe Wanderers, Oxford United, Portsmouth and Fleetwood Town will contest the play-offs for the final promotion place.
Doncaster Rovers finished ninth in the standings.
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