Yussuf made 101 league appearances while Mitchel-King played 47 games over two seasons and they join Pim Balkestein, Chris Hussey, Stacy Long, Curtis Osano, Jason Prior, Louis Harris and Kelly Youga in departing the club.
And Ardley admitted he made some tough decisions.
"That is the disappointing part of my job because all of them have been hard-working and have had a fantastic attitude," he told the club's official website.
"It's sad, but that's football, and if you are trying to progress and shape the team the way you want it, when these things have to happen.
New contracts have been offered to John Sullivan, Luke Moore, Charlie Strutton and Kevin Sainte-Luce while Mikhael Jaimez-Ruiz will hold discussions with Ardley about his future when he returns from holiday.
"We have made offers to those players because we feel they can play a part in us moving forward," Neal added.