Roper joins Whitewater from Volume where he was director of planning. He previously spent years on the board of agencies MHA Carlson and Banner.
Roper will oversee the direction of over 20 UK leading charity brands including WaterAid, Multiple Sclerosis Society and the Royal Society for the Prevention of Cruelty to Animals.
Steve Andrews, chairman of Whitewater, said "We were privileged to meet many tremendous people but our decision, in the end, was an easy one.
"Mark was an outstanding candidate who we could see taking Whitewater to the next level."
Roper commented "There is a history of excellence to build on with Whitewater and I relish the opportunity of bringing my experience to the team."
Roper's appointment is effective as of October 1.