Ed Upson's late strike gave the Glovers their first ever win in the second tier of English football - although they were made to work hard for the three points.
The game looked as if it was heading for a goalless draw until Upson popped up to hit the winner for the side who were playing non-league football a decade ago.
And Johnson says his side are using the fact that no one is giving them a prayer of staying up as motivation.
He said: "We are used to people telling us we are going to get relegated and with our budget we really should be, but we use it as a strength.
"Anything we do is a bonus. But we have a group of lads who know how to put in a shift and work for each other. They will all improve as they are a young lot.
"I think we deserved that win. I thought we played the best side in the first half without having any real chances.
"We left it a bit late, but it was a nice feeling when the goal goes in that late because they did not have any time to recover.
"The boys are buzzing, but it is just one game. It means nothing if we lose the next 45. So we have to keep our feet on the ground, we have to keep working."
Millwall had the best of the opening chances with the returning Steve Morison going close with a header and then strike partner Andy Keogh saw his glancing effort go wide.
But it was winger James Henry who had the two best chances of the game.
The first before half-time, when he exchanged a neat one-two with Morison on the edge of the area and looked favourite to score, but Yeovil goalkeeper Marek Stech did well to save with his feet.
In the second half, Morison was played in by Nicky Bailey and the ball came to Henry on the edge of the area, but with the goal open he could not keep his effort on target.
Then with just three minutes remaining, Millwall failed to deal with a corner and the ball fell to Upson on the edge of the box and he took one touch before firing past David Forde.
New Millwall manager Steve Lomas said: "We only have ourselves to blame, we had five decent chances and we have to take them.
"It was a typical first game of the season where no one really want to put their foot on the ball.
"But once you don't take your chances and the longer the game goes on you leave yourself open to the sucker punch, and that is what happened.
"From our point of view an unopposed shot on the edge of the area is not good enough.
"I thought it should have been 0-0, but if you don't take your chances then they come back to haunt you."