The Kiwi star shared a throwback photo of the herself with Miller, captioning it: "This was a fun day".
Early reports suggested Miller had died from a suspected drug overdose, and tributes quickly poured in from his friends, fans, and hip-hop peers after the news hit headlines. This is one of my best friends in the whole world and favorite people on the planet, Malcolm McCormick.
The singer ended their two-year-old relationship in May this year and she is now engaged to Pete Davidson.
She guest-starred on his 2016 album "The Divine Feminine", and a year later Miller - sporting his signature baseball cap above his tattooed neck - welcomed Grande on the airport tarmac when she returned to the United States following the deadly attack on her concert in Manchester.
Now, after his death, fans flocked to the post to share their heartbreak over the sudden passing of their idol, who was found in his home on Friday. Later, after a fan suggested their breakup was responsible for Miller's relapse, Grande responded, "I am not a babysitter or a mother and no woman should feel that they need to be".
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'Rest in your peace Mac Miller, ' Solange wrote.
Grande and Miller had dated for about two years before breaking up earlier this year. He told the crowd: "I swear, life is so f***ing fragile".
The singer shared a sweet picture where the rapper is both looking at her and the sky as he sits on the grass. Neither side revealed his cause of death, but the celebrity news site TMZ, which first reported the death, cited unnamed sources as saying he had overdosed on drugs.
"I have cared for him and tried to support his sobriety and prayed for his balance for years (and always will of course) but shaming / blaming women for a man's inability to keep his sh- together is a very major problem", she continued.
"The music artist known as Mac Miller died today in his Studio City home", the LA County Department of Medical Examiner-Coroner said in a statement on Friday.