Andy Keogh's first-half strike means the Iron now cannot be caught by any of the bottom four, despite Will Hoskins' equaliser for the Millers.
It means United's league position this season is guaranteed to be their highest since the mid 60s.
Walsall have been relegated to League Two after their 3-1 home loss to Huddersfield. They now trail fifth-bottom side by seven points, with just six left to play for.
Carlisle United sealed promotion to League One with a 1-1 draw at Mansfield enough to see them up, as Leyton Orient were held at Lincoln City.