The 19-year-old forward entered the fray with the Rams two-goals down to Leeds United, and with just over 20 minutes left to play.
He missed a glorious opportunity from close range to reduce the deficit with 10 minutes remaining, screwing a chance over the bar after Craig Bryson had worked the ball back to him, but his performance showed promising signs.
Having joint the Rams academy at Under-12 level, the young forward had endured a couple of tough years before breaking through last term, and being name Under-21 Player of the Year 2013/14. He has continued his rise in the youth ranks, spearheading their Under-21 side again this year, and winning international recognition - with six England Under-20 caps won since his debut at the beginning of September this year.
He has one goal in those six youth caps, and one goal in seven Under-21 appearances for the Rams this term, and fans will be pleased to see another promising youth prospect make their debut for the club, with the likes of Will Hughes and Mason Bennett already making waves in the first-team.
Despite falling to defeat to Leeds, the youngster was delighted to have made his debut, and he told the official Derby website that it is a memory to be cherished forever.
“It was a really good experience; it has been something that I have been hoping for since I joined the Club in the Under 12s,” Thomas said.
“To make my debut was a great experience and great memory for me and my family.”
Thomas admitted that he was annoyed at himself for missing the chance, but he accepted that not everything can go your way as a player.
He added: “At the time, I was very disappointed [to have missed the chance] because it was my debut and it would have been a dream to add to the occasion.
“When I thought about it after, it is part and parcel of being a striker. You miss some and you score some.
“It was replayed a few times in my mind that night, but as a striker you just have to move on to the next chance and try and put that one away.”