Google says he is, IMDB says he isn't. I can't see him. Anyone know for sure? Pre production news said he was joining the cast, so if he's not in the film where did he go? Did he film any scenes or was he cut before photography?
I found this IMDb article mentioning "Jason Isaacs joins the cast of John Wick." At the bottom of the article, it also mentions Isaacs had joined the cast of the Brad Pitt vehicle, Fury (which IMDb's cast listing confirms he was a part of.)
Now, what follows is purely supposition on my part, as I couldn't find any direct articles about it, but I do notice that both Fury and John Wick were doing principal filmography between September and December 2013, and in very different parts of the world.
Principal photography began on September 30, 2013, in the Oxfordshire countryside... Filming was wrapped up on November 15, 2013 in Oxfordshire.
For John Wick:
On 14 October 2013, shooting began in Mill Neck, New York, with the filming process scheduled to continue in and around New York City and greater New York area... Principal photography was confirmed to have ended on December 20, 2013
So, I suspect that Isaacs had to drop one of the movies due to scheduling conflicts, and evidently he chose to drop John Wick. (Perhaps because Fury had already started filming before the other, but again, this is just me putting 2 + 2 together, without a direct reference.)