Jackie McNamara's side suffered their first defeat in three games as the Buddies notched up a 4-1 win.
Saints also fielded youth team graduates Sean Kelly and Kenny McLean against the Arabs - who finished with 10 men after Keith Watson's red card - and Lennon spoke with pride of their recent development.
He said: "It was a fantastic result for us. A lot is spoken about the Dundee United youngsters and written about them, and rightly so.
"But I am delighted with the brilliant crop of youngsters we have.
"None more so than wee McGinn for the composure and finesse that he showed for the second goal. Also the hunger and desire from Naismith for the third goal. Absolutely fantastic.
"The evidence is there that they can go far.
"The United lads get the credit because of the position they play. They are flair players who score goals.
"But we have a fantastic conveyor belt here and I'm delighted that they are becoming good impact players."
Thompson slotted St Mirren ahead after 12 minutes from Conor Newton's knock down, while McGinn slalomed his way into the box before slipping in the second.
Lennon had to sub skipper Jim Goodwin before he was sent off but it was United who finished with 10 men when Watson saw red for a crude tackle on Kelly before Naismith prodded home the third from Paul McGowan's through ball.
McGowan teed up Thompson for the fourth but McNamara is not concerned the heavy defeat will stunt his youngst olayers' growth.
Asked if he was worried about the continuing hype surrounding his side, the United boss said: "The hype hasn't come from us. It has been people watching us.
"The hype is through the media and it is nice to get the recognition but we won't get too carried away. We'll just keep playing the same way.
"Today we tried it and certain things didn't go for us.
"I'm disappointed to lose the match. A few things went against us.
"The lads kept at it though and tried to keep playing football and creating chances.
"Their keeper has had three or four tremendous saves. (Gary) Mackay-Steven has seen him make a good save and Nadir (Ciftci) had another at the back post."