The Lions went into the game with only one win from their last nine in all competitions but deservedly went in front after 11 minutes through a fine 20-yard effort from Morison.
Derby rarely threatened and, four minutes after half-time, Morison turned provider with a pinpoint cross for Theo Robinson to bullet home a diving header.
The Rams tried to find a way back into the game and Shaun Barker went close with a header but the Millwall defence stood firm.
The win ended a run of five games without a home win for Millwall, who moved above Derby and into ninth place in the npower Championship.
Source: PA
