The duo, who were installed as joint caretaker-managers following the departure of Owen Coyle, will oversee all football operations at Victoria Park, including the new Reserve team and the club’s youth teams.
A new Academy Director will be brought in to succeed Ferguson, who was previously fulfilling the role, while Richard Brittain has been named Reserve team boss.
County chairman Roy McGregor said he was delighted to appoint the pair.
Describing Ferguson and Kettlewell as “a perfect fit”, he added: “No one knows the club and playing staff - from first team down to our youngest age group - as well as them.
“They are relishing the opportunity to take on their roles permanently.
“Steven is one of the most qualified coaches in the country, holding his UEFA Pro License, and, having played here too and spent years establishing our current youth set up, knows the club inside out.
“Stuart is one of the best young coaches in Scotland and - as well as having first team coaching experience under the last two managers - last season he led the Development Squad to an historic league title.
“In their short time in charge it has become obvious that they could not be overlooked in the search for a permanent manager.
Ferguson added: “I’m delighted to become permanent co-manager of Ross County. It’s a fantastic opportunity to have the chance to work with the first team - as well as overseeing all age groups of football at the club - and it’s one that neither myself or Stuart could turn down.
“Working alongside Stuart felt right as soon as we started and it was really important to both of us in accepting the permanent job that we could have a say in the Academy and Reserves.”
Kettlewell added: “There’s no one else I’d want to do it with more than Steven. We work well together and the club means a lot to both of us.
“We are a different football club and because of that sometimes we have to do things differently from other clubs. But we firmly believe it to be right for the club.
“Needless to say, our permanent appointment brings some clarity to all the coaching and playing staff so they know what direction the club is moving in in the future.
“We couldn’t have asked for more from the players since we took over. I think they have all bought in to what myself and Steven have been trying to do since we took charge and we look forward to working with them all going forward.”