Frank Lampard's Derby side needed a victory to guarantee sixth place and they could start celebrating after two goals in three minutes in the second half from Mason Bennett and Harry Wilson settled the jitters.
West Brom, already assured of fourth spot, had equalised just after halftime through Stefan Johansen after Derby had led at the interval through Martyn Waghorn's header.
Middlesbrough did their part, winning 2-1 away at Rotherham United while Bristol City, who also still had a chance of making the top six if results went their way, drew 1-1 at Hull City.
"The lads deserved it and we played brilliantly in spells but when it goes back to 1-1 you fear, I knew Middlesbrough were winning," Chelsea great Lampard told Sky Sports.
"They were magnificent today. I said be brave, there was a lot on this game."
Norwich City were confirmed as champions after winning 2-1 away at fifth-placed Aston Villa. Sheffield United, who finished runners-up and also gained automatic promotion to the Premier League, ended with a 2-2 draw at Stoke City.
Third-placed Leeds United, who for so long seemed destined to go up automatically before a late-season slump, finished their regular season with a surprise 3-2 defeat by bottom club Ipswich Town.
It means Leeds and Derby will face each other in a two-legged playoff semi-final clash with added spice after the so called "spy-gate" saga in which Leeds officials were caught at Derby's training ground before the sides met in January.
Head coach Marcelo Bielsa subsequently admitted he had spied on all of Leeds' opponents in a similar manner.
Leeds were fined 200,000 pounds by the Football League -- a fine Bielsa has since said he paid from his own pocket.
The other semi-final will be an all-Midlands affair between Aston Villa and West Bromwich Albion.
The winners of the two playoff semi-finals will face each other on May 26 in what is widely-regarded as the most lucrative match in world soccer thanks to the Premier League's astronomical television deals.
(Reporting by Martyn Herman; Editing by Toby Davis)
