Bradshaw produced a quality finish to extend Millwall's unbeaten run to seven matches and send Derby crashing to their first home defeat since late August.
It was another disappointing display from Derby, who created little until the second half when Martyn Waghorn twice went close but Millwall were resolute and deserved to take the points.
Millwall boss Rowett was among a number of ex-Derby players who laid a wreath in the centre circle in memory of former manager Jim Smith, who died on Tuesday.
Rowett took Derby to the play-off semi-finals two seasons ago but he returned to Pride Park in charge of a Millwall side unbeaten in six and they had a great chance to go ahead in the 12th minute.
Another quick Millwall raid three minutes later saw Mahlon Romeo whip in a cross which Bradshaw just failed to connect with but they got the goal they deserved in the 25th minute.
Shaun Hutchinson lifted a ball over the top and Bradshaw headed it across Craig Forsyth before driving a shot into the top-left corner of Hamer's net.
Derby looked flat and short of confidence and there was another anxious moment in the 42nd minute when Jed Wallace's shot from outside the area deflected behind off a Millwall player.
Boos greeted the half-time whistle which was no surprise after such a tepid display from Derby but they started the second with more urgency, creating a chance for Waghorn in the 49th minute which he put over from 10 yards.
Derby were moving the ball with more purpose and Millwall were coming under pressure for the first time as Waghorn tested Bartosz Bialkowski with a 25-yard shot before the keeper pushed out a Jayden Bogle cross.
Millwall were forced to drop deeper and Waghorn went close again in the 76th minute when he flashed a shot just wide from a Curtis Davies cross.
Bialkowski showed good reactions when he turned behind a deflected Bogle shot three minutes later but Millwall wasted a good opening when Romeo lost control after breaking into the box in the 84th minute.
Millwall had defended well and although five minutes of stoppage time was played, Derby could not break them down as the Lions repeated last season's victory at Pride Park.