Dougie Freedman had been in upbeat mood throughout the week but his hopes of a City Ground send-off to take Reds fans into the summer with a spring in their step was undone by two first-half Cardiff goals.
Joe Ralls (14) and Eoin Doyle (24) netted for the visitors, who went into the game level on points with Forest.
Chuba Akpom was sent on at half-time for his last Forest appearance, replacing Gary Gardner for an inauspicious end to the Villa loanee's time with the club.
Two further substitutions were made before the Reds were thrown a lifeline as Cardiff goalkeeper David Marshall was shown the red card with 15 minutes remaining.
The expected siege on the opposition goal did arrive but Dexter Blackstock's strike proved to be just a consolation.
It means that the Reds end the campaign with four successive home defeats and no win in their last eight games and leaves Freedman, eternally optimistic about his players over the past few months, with a tough task to paint a positive picture about the summer months to come.