A former Wales Under-18 and Under-20 coach, King previously spent seven successful years at Aberavon.
He led the Port Talbot club to a third-placed finish in the Principality Premiership In 2009 and 2012 and to a runners-up place in the 2011 SWALEC Cup final.
King said: "It's a fantastic opportunity and I'm absolutely delighted to be given this role. I have experience at this level and now look forward to working at a club with such a rich heritage and proud history.
"There is undoubtedly huge potential at Cardiff RFC and if we can tap into that and drive everyone forward we can look forward to a very bright future at the Arms Park.
"There are really good people working in this region and at Cardiff RFC, passionate people who have a real affinity to the Blue and Blacks. I'm looking forward to working with them and taking us forward.
"The players can now look forward to being really challenged. We will challenge individuals and the team group and where they want to go. I'm confident that we can bring the very best out of them all.
"We have a good opportunity at home against Neath on Saturday and then seven more games in the Principality Premiership season and a home quarter-final in the SWALEC Cup to look forward to.There is a lot of rugby for us to get stuck into this season and I look forward to what we can achieve."
Everybody at Cardiff Blues would like to wish Simon King every success in his new role.