Horler left the tapas chain in June after completion of its purchase by Robert Tchenguiz's investment vehicle R20.
His appointment at Leon is his first job in his post La Tasca career. Horler has been an investor in Leon since June 2005.
John Vincent, co-founder of Leon, told Caterer: "James has been a personal investor in the business and we were delighted when the opportunity came up for him to become chairman."
"James has enormous experience and is a great believer in what Leon does. He is a fantastic character with the experience at the top and the business acumen to be a great asset to Leon," said Vincent.
Horler said he was a huge believer in the Leon vision and looked forward to working with the team to grow the business.
Horler bought La Tasca in September 2001 for £28.2m with backing from private equity firm Penta Capital. It was incorporated into the Laurel Pub Company in June after being bought by its private backers R20 for £123m in April.
By Christopher Walton
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