Major Charles Ingram, who used coded coughs to answer questions on the quiz, was sacked on Thursday.
"I very much regret that the pressures associated with recent events have made it impossible for me to complete my full term of military service as I had hitherto envisaged," he said.
"But my service in the army is and will remain a source of pride."
Guilty of deception
The Bosnian veteran received a letter on Thursday morning from the army board of inquiry, telling him to leave by August 20.
A Ministry of Defence spokeswoman said: "An administrative investigation was carried out by the chain of command following the fact that he was found guilty of deception.
"The Army Board has delivered its findings and requested that Major Ingram resign his commission.”
Major Ingram was convicted earlier this year of deception, along with his wife Diana and college lecturer Tecwen Whittock, 53.
"My service in the army is and will remain a source of pride."
- Major Charles Ingram
He cheated his way to the top prize by asking his wife and Whitock to direct him to the right answers by coughing at appropriate moments.
The Ingrams, both 39, were each fined £15,000, ordered to pay £10,000 costs, and received 18-month prison sentences, suspended for two years.
The father-of-three will lose his five-bedroom house, which is currently rented by the army on his behalf, but he will get to keep his pension.
The Ingrams have received various offers of work since their appearance in court, which made headlines around the world.
Although they are banned by law from profiting from their crime in the UK, they could make money from it by working abroad.
The Millionaire gameshow is hugely popular and is used in different formats by TV stations throughout the world.