Beaten 6-0 in the corresponding fixture last term, the home side were soon in control through Brett Pitman's third-minute strike and a 38th-minute goal from Lee Johnson.
Jamal Campbell-Ryce fired his first goal for Bristol to complete the scoring on 55 minutes and it was another defensive horror show by Dave Jones' side.
Millen said: "We have had some bad results against Cardiff in recent seasons and I have been involved as much as anybody so it was great to turn things around.
"I'm particularly pleased for our fans because they have had to take some stick from their Cardiff counterparts. Today we really showed what we are about and it was a really convincing victory."
Source: PA
Source: PA