The former St Johnstone boss - who led Saints to the Scottish Cup and several top-six finishes during his time in charge at McDiarmid Park - has signed a contract until 2023 at the BBSP Stadium, Rugby Park.
The Northern Irishman, whose first game as Killie manager is against Motherwell on Wednesday, told the club website: "I’m absolutely delighted to be the new Kilmarnock manager - it's a great opportunity for me.
“I had nine years in total at St Johnstone, and the time was right from me to leave there, but it is the perfect time for me to come back to management."
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