Goals from debutant Andreas Cornelius, Nicky Maynard, Peter Whittingham and Craig Noone sealed the Premier League new boys' victory against their non-league opponents.
The hosts scored three good goals of their own through James Norwood, Danny Wright and Matty Taylor that are sure to have given Cardiff boss Malky Mackay some cause for concern.
New signing Cornelius looked lively up-front and the Danish striker grabbed his first goal for the club after just 16 minutes. The 20-year-old rose to meet a Kevin McNaughton cross and gave Sam Russell no chance with a powerful header.
The Conference Premier side briefly threatened but Nicky Maynard doubled Cardiff's lead soon after. Kim Bo-Kyung and Craig Conway worked a neat corner routine that led to the striker firing beyond Russell.
However, Forest Green pulled a goal back on the stroke of half-time as Norwood finally got his reward for some good play, the midfielder poking his shot past Joe Lewis' outstretched arm.
Mackay made 11 changes at the break, and his side restored their two-cushion almost immediately after the restart. Joe Mason was brought down inside the penalty area, giving Whittingham the opportunity to score from the spot. The midfielder made no mistake, placing the ball emphatically past Russell.
Danny Wright brought the home side back within one goal with a strike just before the hour, but substitute Noone added a fourth for the visitors with 18 minutes left to play.
The hosts had the final say in the match as Taylor made no mistake after finding himself unmarked inside the penalty area, but Cardiff held on for the win.
