In a statement, Fewell said that he leaves Channel 4 News producer ITN after "the time of my journalistic life", but would now "try something new".
His departure comes just weeks after Channel 4 News editor Jim Gray quit the programme after 15 years, meaning there will be a complete change at the top of the programme.
Ben de Pear, the former head of foreign news at Channel 4 News who was recently appointed Gray's successor as editor, will now look to find both his replacement and a new deputy editor.
This search will commence with immediate effect, said ITN, and de Pear is "expected to make an announcement on a new senior team in the coming weeks".
During his time at Channel 4 News, Fewell was responsible for setting the programme's editorial agenda and leading several periods of change within the newsroom, including the shift to multiplatform coverage.
He led a major review of the programme in 2011, the same year that Channel 4 News won a BAFTA for its coverage. He is also credited with launching Snowmail - the nightly email bulletin from Jon Snow and other presenters.
"I'm immensely proud to have worked with such a strong and committed team at Channel 4 News and to have been deputy to a terrific editor like Jim Gray," said Fewell.
"I'm not sure I can top the experience as a journalist, but what I've learned from a brilliant, creative set of colleagues & friends at ITN will stay with me as I head off to find new opportunities.
"The programme is in great shape, in great hands and it feels like the right time to try something new."
Fewell started his career as a local radio trainee reporter in 1987, but later joined the BBC, becoming deputy editor for The World at One and The World This Weekend. He was launch editor on BBC News 24, and worked on Newsnight and the One O'clock News.
He joined ITN in 1998 as a programme editor for the 7pm Channel 4 bulletin and special editions, before becoming deputy editor in 2002, with his first major initiatives being a week of programmes presented in or around Delhi, India.
ITN's chief executive John Hardie said that Fewell would be "sorely missed" by his colleagues at the news broadcaster.
"He brings absolute professionalism, passion and an intellectual edge to every project he leads," said Hardie.
"He was instrumental in our review of the programme last year, and before that played a key role in developing Channel 4 News as a multimedia brand. We wish him all the best."
Channel 4's head of news and current affairs Dorothy Byrne added: "Martin's contribution to the history of Channel 4 News and the programme it is today has been truly outstanding.
"He has been fantastic to work with and I have relied upon his judgment, expertise and support for many years now. He leaves the programme in brilliant shape, and I wish him all the best for his next challenge."