The 50-year-old replaces Mark Roberts, who was sacked as the club battle against relegation from Scottish League One.
Ayr said McCall had impressed hugely with his “hunger for the challenge, passion and professionalism”.
McCall said: “The club is underachieving and my initial aim is a simple one - to start winning matches.
“My philosophy is always about taking each game on its merits, one week we may be able to play some great football and score a few goals and the next it’s about grinding out a victory.”
Chairman Lachlan Cameron added: “Ian has been reinvigorated and has a proven ability to work at our level and take control of the club.
“He also has a huge passion for players and will make a big difference in the dressing room. His enthusiasm is infectious and said to me he is wanting to take us on a ride.”
Veteran McCall has not managed a team since leaving Thistle in 2011.
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However, he has been looking for a return to the game and is delighted by the prospect of taking over at Ayr. He said: “I took a bit of time out of football after leaving Thistle but I could not stay away.
“I have been keen to get back in for ages now and have had to patiently wait my chance. I have coached teams when I could and kept myself involved in other ways.”
He also spoke about his decision to enlist Neil Lennon and Derek McInnes to help him convince Ayr that he was the right man for the job. He added:
“Neil and Derek knew how much I wanted to be the manager of Ayr and agreed to speak to the people here about how I can lift them away from near the bottom of the table. I am delighted to be given the chance.”
McCall said he “cannot wait to get started” and will take charge for Ayr’s game against Brechin this weekend.
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