Saints' Aviva Premiership title dream died in controversial circumstances at Welford Road.
Tigers centre Tuilagi escaped with a yellow card after punching Northampton's England wing Chris Ashton eight minutes before half-time.
Ashton, who required treatment as a result, was also sin-binned after an incident that reflected a game that rarely dipped below boiling point.
Tigers wing Alesana Tuilagi's late try confirmed a seventh successive Premiership final appearance for Leicester. They will meet Saracens or Gloucester at Twickenham on May 28.
But the 11-3 defeat left Mallinder questioning the decision not to send off 19-year-old Tuilagi junior.
He said: "First of all, you can't have players taking players out, and you can't be hitting players directly to the head, punching.
"Chris Ashton did go over and give him a push, but you cannot react with three punches to the head without a red card.
"It would have had an effect on the game, because they would have been down to 14 men. But Leicester played well in the second half - it is fine lines.
"When you come to places like this, you don't get the 50-50s.
"I think the referee did go to his assistant. All I know is I have seen it, and to me, it is a clear red card. What did Ashton get a yellow card for - pushing?
"It's a big game, live on television and it should be one of the highlights of the Premiership season. I guess the referees and referees' assessors will have a real good look at it and work as hard as they can to make sure it doesn't happen again."
Mallinder must now pick his players up for next Saturday's Heineken Cup final against Irish heavyweights Leinster in Cardiff, when Saints will target landing a first European title since 2000.
"We've lost a close game to a good side," he added.
"We will have a good week, and it's a one-off game next weekend now."
Leicester rugby director Richard Cockerill claimed he did not see the Tuilagi incident, and grew increasingly exasperated about suggestions that it might have been turning point of the semi-final.
He said: "I was watching the ball, but the assistant referee clearly saw it and deemed it as a yellow card offence. I haven't seen the replays of it.
"It was a good performance against a really good side. Let's talk about that shall we? Let's talk about how we controlled the game, let's talk about Alesana's try, let's talk about the scrum contest, not all the crap you boys want to talk about and make headlines out of.
"He (Manu Tuilagi) is a quality young player who does what he does exceptionally well. We will have to see what happens. We are guessing, aren't we?
"Our attitude to defend and not let them get any cheap points, and our tenacity at the breakdown was very good. We controlled the gain-line really well, and that was our best all-round performance for a long time.
"We wanted to put them under pressure. You stop them from getting on the front foot and they are not as effective as they can be."
The Rugby Football Union citing commissioner must now decide whether or not to cite Tuilagi, with any ban likely to sideline him from the Premiership final and possibly rule him out of games in England Saxons' Churchill Cup campaign next month.
Leicester, though, are now one win away from completing the second title hat-trick in Premiership history, having achieved the previous one from 1999 to 2001.
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The Barbarians are one of the top invitational rugby sides with a long history and classic rugby jersey.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Lions get ready for their matches with this green training jersey. It's what the players wear. Get in the Gear!
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The Lions are ready to get their Australia tour underway. They arrive in Perth on Monday.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.