The former Forest Green, Kettering Town, AFC Telford and Tamworth boss is charged with turning the Glovers season around.
Yeovil are now in the bottom four following recent defeats to Oldham and Aldershot.
Those results confirmed the departure of summer appointment Hargreaves and Cooper jumped at the chance to steer the club up the National League table.
His appointment comes just hours before the club's massive match at fellow strugglers Maidstone United on Saturday.
The 53-year-old signs a two-and-a-half-year deal in Somerset.
He told the club’s website: “It’s good to be here. I already know a few of the players and staff so I’m excited and really looking forward to it.
“If you look historically, this can be a big club. We need to unite - the players, the supporters. Everybody needs to come together and that can play such a big part.”
Chairman Scott Priestnall feels this appointment will be a success as the club look to climb away from trouble.
“I’m delighted to welcome Mark to the Club, he is a manager I have admired for a long time,” he said.
“Mark has the promotion credentials we are looking for, with a strong background at this level and above.
“I’m pleased to get the deal done swiftly so Mark can get straight to work.”
PHOTO: PINNACLE