Kohn underwent an exploratory operation at the end of last season and discovered a long-standing shoulder injury was more serious than first thought. The 32-year-old lock was advised by his surgeon not to play again.
Skinner's retirement was less sudden. The 29-year-old flanker has not played since suffering a neck injury at the start of last season.
Both players were part of the Harlequins squad that won the Amlin Challenge Cup in 2011 and the Premiership title in 2012.
Harlequins director of rugby Conor O'Shea said: "We are devastated that injury has forced both Olly and Will to retire.
"They have been and will always be remembered as true greats of this club and have inked their names indelibly into its history.
"Will has battled for the last year with his neck, however, Olly's retirement has come about more suddenly for him and Harlequins.
"The club intends to keep both players involved in different capacities as part of its future plans. I have no doubt that, given the young talent that we have, both men will be valuable in mentoring our development players for years to come.
"Day-to-day we are going to miss their personalities greatly, but they will still be around the squad as the club supports them with physiotherapy to alleviate painful aspects of their injuries, before helping them to focus on other areas post-rugby."
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