He struck a 54th minute penalty after Cameron Jerome had been brought down and then netted his second five minutes from time when he knocked in from close range after the visitors had failed to clear a Graham Kavanagh corner.
There was still time for late drama as Forest were reduced to ten men when Jon Olav Hjelde was dismissed for elbowing the impressive Jerome and with almost the last kick of the match City skipper Kavanagh beat Paul Gerrard with a superb shot to complete an outstanding victory.
The Bluebirds kept the side victorious at Derby on New Year's Day while Forest had Hjelde in place of the suspended Michael Dawson.
Cardiff should have opened the scoring midway through the opening period when Jerome beat two men in a run down the left, but his low cross into the danger zone was completely missed by Jobi McAnuff and the chance was lost.
Forest almost made the Bluebirds pay in the 34th minute when Andy Reid's goalbound shot was blocked by Richard Langley.
Forest's caretaker boss Mick Harford brought David Johnson in to lead the visitors' frontline for the second half in place of one-time Bluebirds loanee Neil Harris.
It failed to lift the relegation threatened visitors and it was Cardiff who took the lead when Thorne despatched the spot-kick after John Thompson had floored Jerome.
Forest used all their substitutes to get back on level terms, but it was City who came closest to scoring when Jerome burst past Wes Morgan and crashed in a low shot that Gerrard grabbed at the second attempt.
Paul Parry replaced Junichi Inamoto in the 76th minute, but four minutes later he left the field on a stretcher after been chopped down by Mathieu Louis-Jean.
City wrapped it up five minutes from time when the Forest defence failed to clear a corner and Thorne was on hand to prod home from close range.
In the final minute Hjelde was dismissed and Kavanagh capped a superb performance with City's third goal on the final whistle.