The 35-year-old suffered his first Twickenham defeat since the 1999 World Cup in Tigers' 39-14 Zurich Premiership final thumping against triple champions Wasps.
Johnson led England's imperious unbeaten home run between 1999 and 2003, when they reeled off 21 successive Test match victories following a damaging pool stage reversal against New Zealand.
He can still reflect on a remarkable career that brought him every major success in the game, including a World Cup triumph and three Lions tours - an unprecedented two as skipper.
Johnson admitted he had still not yet fully absorbed the full implications of his decision to retire.
He told BBC Radio 5 Live: "Not really now - it is difficult to comprehend.
"We are at the business end of the season - we have just been concentrating on the game.
"It was so long - 16 years at Leicester, 18 years in all - it is hard to comprehend I won't be there next year.
"But its time to go. It's 100 per cent the right decision - it is all done."
For Johnson, a legendary figure far more used to winning than losing, defeat to Wasps proved a horribly hollow experience.
"I am very disappointed, not because it was my last game, but because we (Leicester) didn't do it. We didn't play, and it's a nasty feeling," he said.
"We beat Wasps a few weeks ago, but you have got to win at Twickenham to be champions.
"We've got to stand up and say they are the better team. They deserved to win because they played better on the day.
"We probably had more territory than them in the first half, and had two or three chances, but we didn't take them," Johnson added.
"We went behind, and their defence was very good. They also kicked very well to pin us back. We would do something good, and then do something bad and lose the ball, whereas they had the momentum all day."
Johnson will return to Twickenham for his testimonial match on June 4, the last time he appears at English rugby headquarters, and a bumper crowd is expected to bid him farewell.