City were without star player Tim Cahill, who watched from the stands with an Achilles injury, but Fornaroli more than made up for his absence opening the scoring in stunning fashion in the 11th minute.
Adelaide goalkeeper Eugene Galekovic played a risky pass to full-back Tarek Elrich, good pressure from City's Luke Brattan and Fernando Brandan saw the ball spill to Fornaroli who was waiting on the edge of the penalty box.
The Uruguayan striker produced a beautiful curling shot with his right-foot to bend the ball past Galekovic and into the top-right corner of the goal.
City, who made four changes after the mid-week win over Melbourne Victory in the FFA Cup, were in control for the majority of the match.
John Van't Schip's team was perhaps lucky to finish with 11 men on the field, after both Neil Kilkenny and Brandan caused controversy at different points in the match.
Fornaroli doubled his side's lead in the 56th minute when he converted a penalty with a Panenka style chip, after Sergi Guardiola had been called for a handball in the box.
The Adelaide striker did manage to make some amends when he headed home a wonderful Ben Garuccio cross in the 64th minute.
The Reds are still without a win after four matches, while City move, temporarily at least, into second place behind Sydney FC on goal difference.
