Goalkeeping coach Martyn Margetson has also agreed a new contract to stay at the Cardiff City Stadium.
“We are delighted to bring people with the experience Dr Ryland and Lennie have to the club and hope that they can play an important role in helping us achieve our ambition of promotion back to the Premier League,” chief executive Ken Choo said.
“We have put together a strong backroom coaching team this summer which we believe will help maximise the potential within our playing staff.”
Head coach Paul Trollope added: “I am delighted to finalise my backroom staff ahead of the forthcoming season.
“We’ve worked very hard over the past month to secure their services and I feel it’s a highly experienced, driven and ambitious staff.”