Phil Jackson never had full support from his general manager Steve Mills — one factor in his Knicks demise — according to one of Jackson’s longtime biographers/confidants.

Jackson is back in Montana after a long trip to Europe and spoke this week with Roland Lazenby, who authored the Jackson biography, “Mindgames,’’ and most recently Kobe Bryant’s definitive biography. Jackson did not show up for Bryant’s jersey retirement ceremony Monday, Lazenby said, because those ceremonies “aren’t his thing.”

Lazenby gives Jackson a lot of the blame for his presidential failure, but revealed that Mills wasn’t in his corner enough regarding his beliefs.

“Phil’s people felt he never had true control in NY, not with Steve Mills there,’’ Lazenby told The Post on Tuesday. “It was the owner’s (James Dolan) way of keeping control. But the failure belongs to Phil because he didn’t find a way to get it done. Too bad Phil never got to coach in New York. It was physically impossible, but as a coach he might have been better able to help Carmelo [Anthony] understand what was required of him.”

Lazenby made his comment before Mills told the AP on Wednesday that he tried to change Jackson’s ways because he saw the franchise sliding in the wrong direction. Mills survived Jackson’s firing and was promoted to president and is presiding over a 17-14 start.

“Everyone was frustrated,’’ Mills said of last season. “One of our players (Carmelo Anthony) was obviously frustrated. Our fans were frustrated, we were frustrated, and so it led us to think we have to do something different and I felt strongly about it. I addressed it with Phil and our coaching staff and our entire staff, that in my view we weren’t a team that really stood for anything in particular and that needed to change.

“If it meant changing the triangle, it if meant changing our day-to-day stuff, we had to become more definable by something.”

Mills’ ambivalence toward the triangle was well known. The Post reported during Jackson’s initial coaching search — after Steve Kerr reneged on a verbal commitment — Mills tried to get the Zen Master to interview non-triangle coaches, specifically Mark Jackson.