The former Metz forward repaid the faith of his manager after just eight minutes when Craig Noone beat Shane Lowry with a mazy run before crossing for the Frenchman who tapped home from six yards.
Millwall were showing no lack of firepower themselves following 11-goal Chris Wood's return to parent club West Brom. Lowry delivered for Darius Henderson after 24 minutes and his header drew a fine stop from David Marshall.
Cardiff had Marshall to thank again when he blocked a stinging drive from Dan Shittu with his feet to ensure they led at the break.
Substitute Jordon Mutch went close to a second 10 minutes after the restart but David Forde produced a flying save to keep out his long-range drive.
Dangerous in attack, the hosts looked susceptible at the back and Dany N'Guessan, recalled to the starting XI following the departure of Wood, fired a warning shot into the side-netting on the hour mark after a swift counter-attack.
Kenny Jackett's side were firmly on top in the second half and the impressive Marshall was again called into action minutes later, this time tipping James Henry's free-kick around the post.
His opposite number was keen to prove his worth as the game began to open up in the final 20 minutes and Forde kept them in it with a brilliant parry to deny Noone.
Malky Mackay introduced Heidar Helguson in place of Gestede 13 minutes from time as the Bluebirds used all their experience to hold on for a vital three points.
- Sports & Recreation
- David Forde
- Darius Henderson