Cancer survivor Graham took over the KM Group-owned Mercury, which celebrated its 150th anniversary last year, in 1998.
He will be succeeded by its current chief reporter Beth Robson.
Said Graham: “Doing this job has been a real pleasure and a privilege. East Kent is a fabulous community to serve, full of wonderful people.
“I have loved it here, which is why I have been here for far longer than I have in any other job in my career.”
Graham started his career in 1973 on the Acton Gazette in West London, where he comes from, moving to Kent in 1977.
He had spells on the Folkestone Herald and Dover Express, latterly as deputy editor, the Kentish Express and the Kent Evening Post, and was editor of the travel trade publication Travel GBI for four years.
Before joining the Mercury, he was editor of the Whitstable/Herne Bay Times.
The biggest stories covered by the Mercury in Graham’s time have been Deal Town FC winning the FA Vase – the only Kent football club to do so – in 2000, and Pfizer announcing it was pulling out of its Sandwich base in 2011.
Graham underwent an operation for prostate cancer at Kent and Canterbury Hospital in September. It was his second operation for cancer at the hospital, following surgery for bowel cancer in 2008.
The keen walker now hopes to undertake more fundraising for PSA (Prostate Cancer Support Association) Kent, which was the Mercury’s charity of the year in 2015.
Graham said: “Both my cancers were detected early, the operations went well and I’ve recovered fully. So I’ve been very, very lucky.
“PSA Kent does a marvellous job and needs money. Ben Eddy, the surgeon at K&C who operated on me, is a tremendously proactive guy who is involved with PSA Kent. After what he’s done for me, it’s the least I can do for him.”