Millwall secured their Championship status for a fourth successive season - but not after going through an emotional rollercoaster in a 1-0 defeat at Pride Park.
Rams sub Conor Sammon slid home an 84th-minute winner - and it was only by virtue of Crystal Palace providing a neighbourly helping hand with a 3-2 success against Peterborough United and Barnsley dropping points at Huddersfield - that Millwall avoided the drop.
Lions assistant boss Joe Gallen, speaking at the end of a long, at times difficult and exhausting season, said: "I don't think that there will be any celebrating tonight.
The plan was for us to come and get at least a point but we ended up relying on other scorelines and it was enough to sneak us over the line.
We've basically run out of energy. We've put out a team that has been solid defensively, but when you play 10 games in 33 days, an FA Cup Semi Final, the fall-out after that game - it's been a tough period for the players and fatigue has hit.
Once Derby scored we were asking what the other scores were. Again, look at today, and the season overall, we've got 56 points - one less than last season.
For a club our size, it's an achievement for us to stay up, not lose a London derby this season and get to an FA Cup Semi Final.
But we know that there is work to do and what departments need strengthening for next season, although if you look at Derby today, they've got a sub costing, what, £1.2million coming off the bench. Offensive players do cost a lot of money."
Saturday's match at Pride Park was certainly not one for the faint-hearted with The Lions, then The Rams, enjoying spells of pressure that could easily have seen the deadlock broken long before Sammon's late strike.
Millwall almost caught Derby on the break after just three minutes when Shaun Batt and Andy Keogh combined to set up Liam Trotter, but a combination of John Brayford and Richard Keogh blocked the midfielder's low shot.
Liam Feeney was denied by the quick reactions of keeper Adam Legzdins, who made amends for a poor clearance that hit Jake Buxton by saving well at the feet of the winger.
At the other end it took a magnificent save by David Forde to push away Keogh's goalbound header from Jamie Ward's free-kick just before the end of the half to ensure the teams went in 0-0 at the break.
The Lions enjoyed a great spell of second-half pressure soon after the restart and it took a goal-line clearance by Paul Coutts to clear an acrobatic overhead kick by Jermaine Easter.
Keogh's near-post header was diverted just over off Brayford as Millwall pressed and then Easter's fierce drive was deflected narrowly off target as the visitors pressed for the opener.
Left-back Shane Lowry hit an unstoppable 35-yarder that flew inches over the bar and Trotter almost turned in a corner from the right from close range as Millwall looked for that crucial opening goal.
But it was Derby who made the all important breakthrough with just six minutes remaining when Sammon exchanged passes with Ward before hitting a low shot under Forde from 12 yards.
The Lions players, at that stage not knowing what the scores were from around the country, went for broke in a bid to get the goal that would have secured safety regardless of other results.
But the goal was not forthcoming and it was only after the final whistle that the Millwall players, huddled on the pitch waiting anxiously for word to come through about Peterborough United, Huddersfield and Barnsley, were able to breathe easy as Championship football was finally, agonisingly, secured for another year.
Derby County: Legzdins, Brayford, R Keogh, Buxton, Roberts, Coutts (Davies, 88mins), Hendrick, Hughes, Bryson, Martin (Sammon, 80mins), Ward (Freeman, 90+1min). Subs not useds: Fielding, Hanson, Jacobs, Bennett.
Scorer: Sammon (84mins)
Millwall: Forde, Dunne, Shittu, Beevers, Lowry, St Ledger (Woolford, 89mins), Trotter, Wright (Abdou, 51mins), Feeney, A Keogh, Batt (Easter, 22mins). Subs not used: M Taylor, A Smith, Osborne, Marquis.
Ref: R Madley (West Yorkshire)
Att: 25,021 (away 1,054)