The rocker posted a three-page handwritten letter on his website on Monday (27Jul15) announcing his departure.
The note read: "It is with a very heavy heart and yet great pride that I announce that I've decided to close this chapter of my life and encapsulate the wonderful times that I've shared with GUNS N' ROSES into fond memories.
"I have reached a point in my life where I feel it's time to dedicate myself to my band Sixx:A.M., my adoring wife and family, and to the many new adventures that the future holds for me."
Ashba, who was recruited by frontman Axl Rose to replace guitarist Robin Finck in 2009, went on to share his gratitude to the singer, writing, "As I sit here writing this letter, I want to take a moment to thank Axl from the bottom of my heart for not only the most incredible experience of my life, but for truly believing in me!
"You are a true friend & a champion of mine & I am forever grateful. And even as I move on to pursue other endeavors, I will always remain one of your biggest fans.
"I also would like to thank each n' every one of you fans for your love & support throughout the years... I hope that you all join me as I forge ahead! Let's conquer this world together!!"
Ashba was the 10th guitarist for Guns N' Roses since forming 30 years ago, although he never performed on an album.
He plans on continuing his work with Motley Crue bassist Nikki Sixx and singer James Michael in the band Sixx:A.M.