Scottish League One club Cove Rangers have signed the 19-year-old goalkeeper from top flight champions Celtic on a season-long loan deal.
Ex-Hoops star Hartley, who currently manages the Toonsers, is grateful to his former club for letting the talented goalkeeper join the side.
The 43-year-old feels Mullen is one of the most talented Scottish goalkeepers currently and is looking forward to providing the youngster with a platform to boost his senior career.
Hartley is delighted to have the Celtic loanee competing with 32-year-old Stuart McKenzie for a starting place and believes the youngster can learn a lot from the senior goalkeeper.
“We are grateful to Celtic for giving us the opportunity to work with Ryan“, Hartley told the club’s official site.
“He is one of the most talented young Scottish goalkeepers in the game and joining us gives him the chance to push his career on a bit.
“It will be good for us to have Ryan competing with Stuart McKenzie for a starting place.
“Ryan will also benefit from working alongside Stuart and our new goalkeeping coach Alan Combe.
“We are very happy to get Ryan on board as we continue our build-up to the new season.”
Mullen will be looking to get valuable first-team playing time under the management of Hartley this season as looks to boost his progress.