In a blog post, Steve Whitmire said he still felt "at the top of his game".
He said he was "devastated to have failed in my duty to my hero" - a reference to late Muppets creator Jim Henson.
Whitmire took over as Kermit's handler and voice actor after Henson died in 1990. No reason has been given for the 57-year-old puppeteer's sacking.
In a statement, the Disney-owned Muppet Studios thanked Whitmore for his "tremendous contributions" and wished him well "in his future endeavours".
In his post, Whitmire said he had "remained silent the last nine months in hopes that the Disney company might reverse their course".
He attributed the change of casting to "two stated issues" and "concerns" raised by executives at The Muppets Studio, without elaborating further.
"I feel that I am at the top of my game, and I want all of you who love the Muppets to know that I would never consider abandoning Kermit or any of the others because to do so would be to forsake the assignment entrusted to me by Jim Henson, my friend and mentor, but even more, my hero," he wrote.
He said the Muppets were not "just a job, or a career, or even a passion" but "a calling, an urgent, undeniable, impossible to resist way of life".
He told fans: "I am sorry if I have disappointed any of you at any point. I am devastated to have failed in my duty to my hero."
Though Whitmire's open letter to his "dear friends" has been reprinted in various outlets, his original blog post no longer appears to be accessible.
Whitmire has worked with the Muppets since 1978 and also provided the voice of Sesame Street's Ernie, of Bert and Ernie fame.
Matt Vogel, a seasoned Muppet performer who used to assist Whitmire while he was playing Ernie, will take over as Kermit from next week.
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The man who has provided the voice of Kermit the Frog since 1990 has said he is "devastated" at losing the role.