The Scotland international - a £750,000 purchase from Celtic during the January transfer window - slid home the winner from Giles Barnes' centre midway through the second half as Derby ended a five-year exile from the top-flight in an entertaining final at Wembley.
Derby boss Billy Davies surprisingly opted to start Paul Peschisolido in attack instead of Jon Macken and the veteran striker should have put his side into a 10th minute lead.
Steven Howard flicked on a long ball from the goalkeeper but Peschisolido hurriedly stabbed a weak effort straight at Dean Kiely.
At the other end, Tyrone Mears saved the Rams as he executed a perfectly-timed tackle to nudge the ball away from Jason Koumas in the penalty area to end a mazy run from the Albion dangerman.
Zoltan Gera was denied as his goalbound volley struck the body of Jay McEveley in the six-yard box as Derby held on at times during a first-half bossed by Tony Mowbray's side.
Kevin Phillips was next to try his luck in the 44th minute as he grazed the crossbar with a rising shot from the edge of the penalty area following a neat move led by Diomansy Kamara and Gera.
Derby grew in confidence after the interval and Kiely produced a breath-taking save to palm Matty Oakley's close-range snap-shot over the crossbar on the hour-mark.
And one minute later Pearson struck to settle the match as he slid the ball home from six yards after beating marker Paul McShane to substitute Barnes' low centre from the right.
West Brom launched wave after wave of attacks in the closing minutes but Bywater wasn't seriously tested as time ticked away on the Baggies' dream of an immediate return to the Premier League.
Even when they did force a glimpse of goal, Mears was again in the right place at the right time in the 86th minute to nudge the ball away from Darren Carter with a perfectly-timed tackle in the penalty area.