While some feared the Cottagers may struggle with a hangover from their midweek European exertions, they impressed from the offset and dominated throughout against their neighbours from less than three miles up the road.
Johnson handed the home side the lead after just 79 seconds and then scored either side of half-time on the way to a resounding win.
"I scored a hat-trick in the Championship but that's my first in the Premier League and a great day for me and my family," he said.
"Strikers hitting 25 or 30 goals is getting harder to do and hat-tricks are far and few between. If you get into double figures these days you have done very well.
"This win meant an awful lot to the fans and the players.
"Everything went in, everything the boys hit which is unfortunate for QPR but they just caught them out.
"We have great players in this team and for the last five or six games we've created so many chances, but keepers have been on form or we've just been unlucky in front of goal.
"However, I think the feeling was that it was due and it was coming.
"Steve Sidwell said before the game that he had a good feeling about today and he came straight off and said 'I told you, I told you'.
"Everyone is delighted. It is a great win and it is great for the fans because it is a local derby as well."
QPR manager Neil Warnock was visibly frustrated by his side's performance on the Craven Cottage touchline and believes the result highlights his side's weaknesses.
"I've had a couple of these results before but it's not easy," he said. "This is one of the worst.
"If ever there was a day to catch us it was today and good players take advantage of a situation. The last 48 hours we've had a nightmare really.
"Danny Gabbidon has had recurrence of his injury, we lost Matt Connolly on Saturday to an innocuous challenge, then Fitz Hall had an injury before the game.
"And you've got Armand Traore sat in the stands [suspension]. We can't cater for that with the squad we've got.
"But I'm learning and at our stage you've got to look after yourselves because things can happen like this and you can lose four players in the space of a few hours.
"It was like a knife slicing through butter. All credit to Fulham, they took advantage.
"I felt at half-time we could've had a go if we had got the next goal and got a bit of pride back but then to concede a goal like the fourth is very disappointing. It's Sunday League stuff."