"I have worked closely with Mark before and there are also players here that I have played with previously, including Bobby Zamora and Shaun Derry," Johnson told www.qpr.co.uk.
"Those were factors in me joining, but this is a big, big club."
Johnson feels he is joining Rangers at an exciting time, the west-London club having secured another season in the top flight on the final day, despite a last-gasp defeat at champions Manchester City.
"I have spoken to the chairman and Mark, who both have great ambition for QPR, and it is something that I wanted to be a part of.
"Mark told me about the plans for a new training ground and how the club wants to go forward. That really excited me."