The 65-year-old's manager Robert Hoy confirmed the news following a story in Southend local paper Echo, revealing that Johnson has chosen not to have any treatment.
"I am very sad to announce that Wilko has recently been diagnosed with terminal cancer of the pancreas," Hoy's statement read.
"He has chosen not to receive any chemotherapy. He is currently in good spirits, is not yet suffering any physical effects and can expect to enjoy at least another few months of reasonable health and activity."
Johnson aims to complete a new CD, tour France and perform a series of farewell gigs in the UK once he returns from a trip to Japan.
Hoy added: "Wilko wishes to offer his sincere thanks for all the support he has had over his long career, from those who have worked with him to, above all, those devoted fans and admirers who have attended his live gigs, bought his recordings and generally made his life such an extraordinarily full and eventful experience. Thank you."
Johnson played mute executioner Ilyn Payne in the first two seasons of Game of Thrones. Before acting, he was part of the original line-up of Dr Feelgood until 1977, during which time they achieved a UK number one album with live recording Stupidity.