The striker pounced after just three minutes to stab in Marvin Elliott's headed knock down and then scored a late spot-kick to reach 20 goals in a season for the first time in his career.
Stephen Pearson's late goal couldn't dampen spirits at the end of a promising week for the hosts, who had already confirmed Steve Coppell as their next manager.
Coppell signed a 12-month rolling contract and has taken on a watching brief from afar due to a pre-planned family excursion to the USA.
When he relives this win on DVD, a wry smile will cross his face, especially with the goalscoring of Maynard at his disposal next term.
City pressed their opponents from the first moments and were rewarded with the former Crewe forward's third-minute strike.
Jamal Campbell-Ryce went close to adding a second goal on 21 minutes with a rasping drive from the edge of the box that flashed inches wide.
Derby's best response came on the stroke of half-time when on-loan Bolton Wanderers man Nicky Hunt was afforded space 30 yards out and a dipping shot was pushed out by returning goalkeeper Dean Gerken.
After the interval, Derby were given a glorious chance to level as Louis Carey's slip allowed Chris Porter to latch on to Stephen Pearson's cross but, with the goal at his mercy, an out-of-sorts Porter fired well wide of the goal.
Maynard sealed victory with 12 minutes left when Hunt tripped him in the box.
With regular penalty-taker Paul Hartley off the pitch, Maynard took the chance to score from the spot and bring about his own special landmark.
Pearson's late strike wasn't enough for Derby as his low finish proved only a consolation.