‘Jonah’, as he is affectionately known throughout the club, was offered the post after the 7-1 victory over a young Cheltenham Town U23 side on the Academy pitch at the Corinium Stadium.
He accepted the role as replacement to Paul Hunt without hesitation.
Mortimer-Jones, 35, is a tremendous club man having played 321 games for Cirencester Town and his appointment will be hugely popular with fellow players and fans.
“Jonah commands a huge amount of respect in the squad and his knowledge of the game and this club in particular is second to none,” said Griffin.
“The boys are buzzing at the moment and I felt that bringing someone in from outside who had not been involved throughout pre-season would not have been ideal.
“Jonah is still a massive player for this club as he showed again in the win over Cheltenham and he will continue to play along with his role as assistant manager.
“We can do this because Rosey (Scott Rose) is so much more than just a goalkeeping coach at the club and very important to us in watching games, looking for new players and scouting the opposition.
“When Jonah has been on the bench with me in pre-season I’ve liked what he has had to say. He’s not a yes man.
“Jonah played 46 games last season and in many of them he was carrying knocks and playing through the pain because we had such a small squad and we were trying to get out of trouble.
“We have more strength in depth this season so maybe we can rest him when we have to.”
Griffin has all but completed his squad building ahead of the new season. Tall teenage wing back/defender Mason Lewis is the latest to join the squad having improved with each game he has played in pre-season.
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