Goalless at the break, the Robins 10-game unbeaten run came to an end courtesy of strikes from Pelly Ruddock, Harry Cornick and an Ashley Williams own goal for their first defeat since the opening day of the season.
"I thought we got beat because of Luton's enthusiasm and too many cheap giveaways from our boys in forward areas and a lackadaisical attitude to release the ball to your man," Johnson said.
"I can count in my head 50 times where we turned the ball over in good areas.
"Against a side that are as energetic and as bright as Luton are, naturally they'll punish you and they punished us and fully deserved their win today.
"Even in the first half, I thought it was a bit too open for both sides.
"We had some good chances. At half time we weren't particularly happy with our defensive work, but quite happy with the work we had done in attacking areas.
"Second half, I just don't think we did the basics well enough, just as simple as that really.
"I don't think we didn't try, or we got beat up, or got bullied, I just don't think we executed enough of our patterns of play and that came down to poor individual quality on the day."
The Hatters took the lead after 56 minutes when Ruddock curled a sublime effort into the top corner from the edge of the box for his first goal of the season.
The second swiftly followed, Brown sending Cornick away who beat the offside trap and then Bentley to make it 2-0 in the 62nd minute.
Luton added gloss to the scoreline in stoppage time when Ryan Tunnicliffe's cross was diverted past Bentley by a full-stretch Williams.
Luton boss Graeme Jones said: "That's four months work there for the players and the staff and I think that was the exhibition of four months work, the evidence.
"The tactical aspect is one thing, the application and the courage and the character of the players to execute it, is really, really satisfying.
"We are capable, we're starting to show that.
"I don't want to make any bold predictions, I said right at the beginning, we want to be competitive in every game and by and large, we have been.
"The biggest thing for the staff and the football club and the players, was that from the last international break, there's been a lot of lessons learned.
"If you don't work hard and you don't go into details, you don't learn lessons, so you keep making the same errors, and that's what we've done.
"Only a fool makes the same mistake twice, so I'm really, really happy with the performance today."