First-choice halves Mitch Moses (calf strain) and Dylan Brown (suspension) watched from afar as breakout fullback Iongi laid on three tries and proved the difference before 8074 fans - the Eels' smallest home crowd since pandemic restrictions.
Sunday night's win will encourage the rebuilding Eels as they face at least three more weeks without Moses, having floundered in his absence to begin the season.
The 16th-placed Eels' win leaves the Titans two competition points adrift in last place.
Making matters worse, Gold Coast will be without halfback Jayden Campbell when they fight to turn things around against an equally desperate North Queensland next week.
Campbell collected Sam Verrills' knee as the pair attempted to tackle Dylan Walker in the first half and was deemed to show category-one head-knock symptoms.
Campbell's absence left goal-kicking duties to second-rower Beau Fermor, who had never kicked a goal at first-grade level before Sunday night.
He managed only one from three attempts, the inaccuracy proving costly as the Titans attempted to fight back from 18-6 down.
Iongi put Sean Russell in for his side's first try with a mammoth cut-out pass down the left side, and had a hand in another when AJ Brimson fumbled his bomb into the arms of Charlie Guymer.
The fullback put Dean Hawkins over in the second half, his grubber kick bouncing from the goalposts and straight to the replacement halfback.
It wasn't all sunshine and rainbows for Iongi. He threw an intercept to Alofiana Khan-Pereira, who belted 90 metres to bring the Titans within two points at the break.
Khan-Pereira's second try made it a one-score game in the second half, before Zac Lomax, on return from NSW duty, snaffled a great offload for Will Penisini to barge over.
Hawkins' one-two with Kelma Tuilagi put Penisini over for a second try only three minutes later as the Eels edged closer to their fifth win of the Jason Ryles era.
Penisini went to the sin bin in the final two minutes for a dangerous throw on Fermor and could face scrutiny from the match review committee.