Mendelssohn opened his account at the second time of asking in the Anglesey Lodge Equine Hospital Irish EBF Maiden at the Curragh.
Aidan O'Brien's Scat Daddy youngster finished down the field behind stablemate The Pentagon on his debut, but was sent off 5-4 favourite to step up markedly on that.
Ryan Moore, fresh from riding in America on Saturday night, attempted to keep it simple and bounced into an early lead.
Everything was going straightforward until the last 100 yards when Mendelssohn, a half-brother to top-class US performer Beholder, drifted markedly to his left, but he still won comfortably from Andesh by a length.
"He was very green, he was green the first day and he's still very green again," said O'Brien.
"He'll probably take another run or two before he clicks into gear mentally.
"Ryan was very happy but he said he was very green, he notices everything. Coming up in front will hopefully have done him the world of good.
"In an ideal world we'd prefer not to go in stakes company if we could and stay a level lower for the minute while he is that green.
"He's a lovely big colt with everything in front of him, but he's just a bit babyish."
Muirin caused a shock in the Loder Irish EBF Fillies Race when turning over O'Brien's Sizzling.
Having never run before, Eddie Lynam's youngster was sent off a 16-1 chance on her debut but there appeared to be no fluke about the victory.
Moore must have thought he had everything covered on 2-5 favourite Sizzling, but she had no answer when Muirin came flying down the outside under Wayne Lordan.
The newcomer won going away by three-quarters of a length.
Lynam said: "We have her in the Moyglare and we like the filly, so you are always dreaming about them a bit.
"Wayne sat on her a few weeks ago, when we brought her away, and he liked her.
"She's in the Moyglare and we'll think about it. Obviously Aidan will have six or seven good fillies to run in that."