Derek Smith joins the council as chief press officer in December, after working at Number 10 for four years. He is to take on specific responsibility for national counter-terrorism and cyber crime in the press, and is also expected to oversee media relations with regards to Afghanistan.
The council was set up last May to oversee all aspects of Britain's security, led by Sir Peter Ricketts. It is chaired by the Prime Minister David Cameron.
Smith formerly worked at the Foreign and Commonwealth Office, where he was press spokesman for former minister Kim Howells.
A recruitment process is under way to find a replacement for Smith in Downing Street.