Johnson, who will draw the curtains on his illustrious career in a few weeks' time, was powerless to prevent a dynamic Toulouse outfit claim victory.
The French aristocrats marched into their third successive European final - they were champions in 1996 and 2003 - and left Johnson's Tigers licking a gaping wound.
Toulouse now face Stade Francais in an all-French Edinburgh final next month. It is hardly a dream ticket for tournament organisers, who saw less than 30,000 tickets sold when Toulouse and Perpignan contested the 2003 final in Dublin.
Flanker Finau Maka and scrum-half Jean-Baptiste Elissalde sent Toulouse on their way with tries early in each half, before skipper Frederic Michalak finished Leicester off by touching down six minutes from time.
Elissalde contributed 17 points, while Leicester could only reflect on four Andy Goode penalties and an injury-time Tom Varndell try, converted by Goode, as paltry compensation for a first Heineken Cup semi-final defeat.
Toulouse had barely 1,000 fans in the 32,000-capacity arena, but they partied long and hard at the final whistle as Leicester were left to concentrate on domestic matters for possible silverware this season.
And just to complete a miserable afternoon, they lost Lions forwards Graham Rowntree and Lewis Moody with injuries, giving Clive Woodward more possible fitness concerns just a month before the team depart for New Zealand. Tigers head coach John Wells said: "Graham hurt his knee and will be out for the rest of the season. He is expected to be back in three or four weeks, so he should okay for the Lions.
"Moody damaged his medial ligament and we believe he will be out for two or three weeks, but we'll just have to wait and see."
To add to Woodward's woes, Bath prop Matt Stevens' hopes of featuring in the tour are in jeopardy after suffering a knee injury yesterday.
The England international took a heavy blow to his right knee during Bath's 21-19 Zurich Premiership victory over London Irish at the Madejski Stadium. Stevens limped off the field after 17 minutes and could face a race against time to be fit when the squad leaves for New Zealand on June 24. Both sides went into battle without key players as they targeted a place at Murrayfield on May 21.
Leicester's England captain Martin Corry was sidelined through suspension - the result of him elbowing Lions colleague Richard Hill during a Zurich Premiership game between Tigers and Saracens - while Toulouse skipper Fabien Pelous missed out due to a knee injury.
Toulouse skipper Frederic Michalak said: "It was a very difficult match, and we found ourselves defending a lot in the first half. We played a lot more rugby in the second half, and we perhaps had luck on our side, and it is a fabulous feeling to qualify for the final in what is a very high level tournament."
Leicester skipper Martin Johnson added: "The match was very cagey and I though Toulouse played a good kicking game in the second half. They kept us pinned back in our own half.
"We could have come back from their second try but when Michalak scored it killed us. We gave them too many opportunities - 3-0 on the try count says it all. We didn't deserve to win."
: Tries: Varndell. Cons: Goode. Pens: Goode 4.
: Murphy, Lloyd, Smith, Gibson, Healey, Goode, Ellis, Rowntree, Chuter, White, M. Johnson, L. Deacon, Moody, Back, H. Tuilagi. Replacements: Morris for Rowntree (19).
: Tries: F. Maka, Elissalde, Michalak. Cons: Elissalde 3. Pens: Elissalde 2.
: Poitrenaud, Clerc, Jauzion, Fritz, Heymans, Michalak, Elissalde, Poux, Servat, Hassanein, Millochlusky, Brennan, Bouihlou, F. Maka, Lamboley. Replacements: I. Maka for Bouihlou (16).
: Alan Rolland (Ireland).