Milner's exit ends a decade-long association with Horizon and he wished the team there well, adding he felt "enormously privileged to have been a part of this exciting journey".
Enthusiastic about the future
"I continue to be encouraged and enthusiastic about the growth prospects of Horizon's business, and intend to remain a shareholder for the foreseeable future," he went on.
Horizon chairman Dr Ian Gilham for his part said he was sorry to see Milner depart, but understood the decision to prioritise "other commitments".
"We are extremely grateful for his valuable insights as a member of Horizon's board over many years and wish him well in his future, not least as a considerable founding shareholder of the Horizon business."