The 76-year-old, who represented Labour, had been on the county council since 2008.
He also served on Castle Morpeth Council between 1986 and 2008 and was Mayor of the former borough twice, in 2005/2006 and 2007/2008.
Coun Douglas served on a number of county council committees – including the economic growth and strategic transport committee and the planning committee, of which he was chairman.
He was also a member of a number of outside bodies, including the Linton and Lynemouth Welfare Management Committees and the North East Regional Employers’ Organisation.
Coun Grant Davey, leader of Northumberland County Council, said: “It is with great sadness we learned of the death of Milburn Douglas over the weekend.
“Milburn was an experienced and highly respected councillor who always put forward the views of the communities he represented – whether at a borough or later a county level.
“He will be sorely missed and the thoughts of everyone at the county council are with his family and friends at this sad time.”
Civic Head councillor Alan Sambrook added: “The best thing I can say about Milburn is that he was a real ‘people person’.
“From the first time I met him, he has been one of the most genuine people you could hope to meet and he will be sadly missed by all of us.”
Lynemouth Parish Council chairman Liz Dunn said: “Dougie, as we all know him, has always represented our ward to the very best of his ability, in an unassuming, practical, no nonsense manner.
“I know I can speak for all members when I say that it was only recently, when ill health caused him to be absent from our parish meetings, that we actually came to appreciate how much he really did work for the people of this parish.
“His absence created a void that will certainly be hard to fill.
“Dougie’s fundamental belief was that he was elected to be a voice for the ordinary people of this area, and by doing that he gained the respect and gratitude of many.”
“As a parish councillor, I will certainly miss his support, his experience, his encouragement and the great depth of local knowledge that he has always been willing to share with others.”
Members of the parish council will be asked to stand in a minute’s silence in memory of Coun Douglas at its meeting this evening.