Leam Richardson's men went into Friday night's contest knowing that only a win would keep their distant chances of survival mathematically alive until at least Saturday's games, having won on Easter Monday to leave them 18 points adrift of 21st-placed Plymouth with 18 points left to fight for.
However, Bali Mumba's strike shortly after the half-hour mark proved to be the only goal of the game, sending Rotherham down while also giving Plymouth a much-needed boost in their own battle to stay in the division.
The Millers offered precious little in reply despite the must-win nature of the match, although their immediate return to League One has long since seemed an inevitability.
For Plymouth, the win is a crucial one as they climb up to 19th in the Championship table, now four points clear of relegation zone with five matches of their season remaining.
Rotherham's inevitable relegation confirmed
While the the thrilling battles for both promotion and to avoid the remaining two relegation places look set to go down to the wire in the Championship this season, Rotherham's fate has long been known.
The Millers had earned themselves a stay of execution in the second tier by beating Millwall on Easter Monday - a first win in any competition since Boxing Day, and one which came unexpectedly following a dire run of just one point from their previous 11 outings before that.
There was never any realistic hope of an Easter-inspired resurrection, though, with Rotherham heading into Friday's match 18 points from safety and with only 18 points left to fight for.
Even a win might not have been enough to keep them clinging on for another weekend if Huddersfield Town or Sheffield Wednesday went on to win on Saturday, but planning can now officially begin for life back in League One next season.
In truth, that planning should have started some time ago, with Rotherham clearly bound for an immediate return to the third tier throughout much of the season, and that has now been confirmed with five games still to play.
Rotherham likely to go down as one of worst-ever Championship teams
In just about every important metric, Rotherham have been comfortably the worst team in the league this season.
First and foremost, their overall points tally stands at just 23 - 16 fewer than the next worst in the division. Even with double their current tally of points, they would still by embroiled in a relegation battle this term.
Indeed, should Rotherham lose their remaining five matches - something by no means out of the question for a team that has been beaten 10 times in their last 13 outings, including a nine-game losing run - then they will equal their own unwanted record of the fewest points ever gained in a Championship season, set in 2016-17.
Only three teams in Championship history have ever failed to hit the 30-point mark; Rotherham are responsible for two of those three teams, and it would take an unlikely sudden improvement for them to avoid that embarrassing fate for a third time.
The Millers' tally of four wins this season is half the number of the next-fewest victories, while unsurprisingly they have lost more games than anyone else too.
Rotherham sit 23rd in terms of goals scored, above only Sheffield Wednesday, but they have conceded a whopping 81 times in 2023-24 - 15 more than any other team in the division.
More to follow.