Since the debut of Super Mario 64 in 1996, the smiling face of Charles Martinet has been almost as iconic as that of Mario himself. First cast by chance in a nondescript audition, Martinet has gone on to voice Mario, Luigi, and almost every other humanoid character in the series in every game since. According to an official Nintendo statement, his time in the spotlight is over. For unspecified reasons, the role will go to a new, as-yet unnamed voice actor in future entries, starting with the upcoming Super Mario Wonder.
Apparently, a future message from Martinet and Shigeru Miyamoto himself will explain things in a bit more depth, but it seems likely that the now 67-year-old Martinet has simply aged out of the role. Apparently, he'll still be kept on as a 'Mario Ambassador', so it doesn't look like Nintendo is done with him just yet.