Both players have worked previously with manager Mark Hughes. Nelsen, 34, was signed by the Welshman at Blackburn Rovers in 2005 while 31-year-old Johnson played for him at Fulham.
Nelsen, who captained the All Whites at the 2010 World Cup finals in South Africa and will lead the Olympic team in London at the Games starting next month, arrives from Tottenham Hotspur on an initial one-year deal.
"I have obviously got a lot of history with the manager and that was a massive draw for me," he told the club website.
"I have got the Olympics coming up and that will be a good way for me to get prepared."
Johnson, who scored a hat-trick in a 6-0 rout of QPR at Craven Cottage in the league last October, was capped eight times by England between 2005 and 2007.
The move from Fulham, on a two-year deal, also reunites him with former strike partner Bobby Zamora.
"Mark was brilliant when I was at Fulham. I'd just come off the back of a rough spell with a knee injury and he guided and helped me through that period," he said.
QPR, run by Malaysian aviation entrepreneur Tony Fernandes, are rebuilding after narrowly escaping relegation last season.
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