Bryn Parry, chief executive of Help for Heroes, is to stand down by the end of the year.
Parry, who founded the charity in 2007 with his wife Emma, said the couple would continue to work as ambassadors for the charity.
In a letter to trustees, published on the Help for Heroes website, Parry says he had intended to run the charity for nine months before returning to his former work as a cartoonist and designer.
He says the time has come to "let others take this wonderful organisation into its next phase".
He says: "The charity is well led, correctly governed, properly funded and in a strong position to face whatever challenges the future may bring, but it needs a different set of skills to mind to take it forward."
Running the charity has been the "most demanding but rewarding period of my life and something of which I will always be immensely proud", says Parry.
"I am a lucky man to have had this opportunity to make such a difference to the lives of others and I am grateful for the privilege, however unexpected and challenging," his letter says.
A response to Parry’s letter from Alex Scott-Barrett, chair of Help for Heroes, says the charity has been an "extraordinary story over the last nine years" and Parry and his wife have been central to its success.
"H4H today is a professional, well-managed charity with a national network of recovery centres," says Scott-Barrett.
"This is a tremendous legacy to leave behind you and something of which I know you are both enormously proud."
He says the charity has started the formal process of finding Parry’s successor.