The club-record signing got off the mark for Nigel Pearson's side as they kept up their unbeaten pre-season run at the New York Stadium with yet another victory
The Argentine - reported to have signed for a club-record fee of £8million when he joined from Brighton and Hove Albion last month - made his debut in a 1-0 win over Everton in Thailand in July, but had yet to register a goal in pre-season prior to Tuesday's fixture.
However, the 28-year-old started brightly against Rotherham and almost sent Matty James through on goal in the opening four minutes.
Rotherham's Ben Pringle saw a 30-yard effort saved by Kasper Schmeichel and Alex Revell glanced a header wide as the Championship outfit pressed for an opener, but it was Leicester who made the breakthrough just before the half-hour mark.
Ulloa reacted well after James had seen an effort blocked in the area and struck across the goalkeeper to open the scoring.
Leicester grabbed a second before the break as Ulloa headed home from a Ritchie De Laet cross.
Kieran Agard pulled one back for Rotherham in the 59th minute after Ryan Hall's effort had deflected into his path, but Danny Drinkwater restored the Premier League new-boys' two-goal advantage by converting a half-volley five minutes later.