Lenagan will step down at the Capital-based club after completing his takeover of the Warriors from Dave Whelan.
Quins have now named Keith Hogg, a former director of the London Broncos, as Chairman with Paul Brown, the chairman of Rugby League Conference side South London Storm, taking over as Chief Executive.
In the short-term Lenagan will remain as the major shareholder at the Stoop, but in accordance with Rugby Football League rules, he has been instructed to hand over all management control and now has a maximum of two years to sell his 65% share.
And Lenagan is convinced that Hogg and new chief executive, Paul Brown, are the men to build on his solid platform.
"I couldn't move away from the club unless I was absolutely certain of the secure and viable future of Harlequins because I have come to love the club," Lenagan said.
"I am committed to keeping my shareholding for a maximum period of two years, meaning I have two years to put together a genuine handing over of the club - not just pass it on to my family.
"I have to use all reasonable endeavours to sell my shares in that two-year period but the management control hands over immediately.
"When I joined the club three years ago I made a long-term commitment to the club and I will maintain my investment until the right owner comes along that has the best interests of the club at heart.
"Financially, Harlequins is sound. It has no debt and there are very few football or rugby clubs that can make a claim like that.
"Keith and Paul may only be here for the interim if a new purchaser comes along, but I would certainly like it to be longer."
The pair will take day-to-day control of the club when Lenagan steps down on December 1.
And Hogg paid tribute to Lenagan for his work during his tenure as Chairman.
"Ian's commitment to Harlequins has been outstanding and we're fortunate that he will continue to be an investor in the club.
"This is a dream come true for me. I have been a long-time supporter of the club and if someone would have said I would be in this seat a few years ago, I wouldn't have believed them.
"Rugby League in London is incredibly important to the sport. I'm really looking forward to next season already - although the Wigan games could be a bit tasty now."