Mycenaean Linear B Tablet, Comparison with the Arcado-Cypriot Linear C “Idalion” Tablet & Implications (Click to ENLARGE): 

Linear B tablet top & Cypriot Linear C Idalion Tablet bottom

[1] While Mycenaean  Greek, written in the Linear B syllabic script, was in use from ca. 1400-1200 BCE, its younger cousin, the Arcado-Cypriot dialect, written in the Linear C syllabic script, subsisted all the way through from ca 1100 BCE to ca 400 BCE (700 years!)
[2] While almost all of the syllabograms the Linear B & Linear C syllabic scripts look entirely unalike, Linear C shares enough syllabograms with its parent, Linear B, for us to draw the sound conclusion that the latter was a much more streamlined and more geometrically simplified script clearly derived from Linear B. To illustrate this, I have flagged the syllabograms on the Linear B tablet above which are (almost) identical with their agnates in Arcdo-Cypriot Linear C.
[3] While these two scripts appear entirely different on the surface, the phonetic-morphemic characteristics & the pronunciation of the latter (Arcado-Cypriot) are so strikingly similar to those of Mycenaean Greek that there is not a shadow of a doubt that the language these two dialects share is practically identical.  Although the pronunciation of Arcado-Cypriot Greek was somewhat, though scarcely greatly, differentiated from that of Mycenaean, these two dialects were clearly offshoots of an even more ancient proto-Greek.
[4] Finally, since Arcado-Cypriot is, without a shadow of a doubt, a Greek dialect, as attested by latter-day texts once it was finally converted into standard alphabetic Greek around 400 BCE, then it necessarily follows that Mycenaean as well must have been a Greek dialect, in other words Greek, and nothing else.