The US rapper was found dead at his home in Los Angeles last week aged 26.
Ariana, who previously shared an image of him on Instagram after his death, broke her silence on the news, describing him as “the kindest, sweetest soul with demons he never deserved”.
She posted a short video on her Instagram account of Mac laughing while the pair had dinner together
She wrote: “I adored you from the day I met you when I was nineteen and I always will.
“I can’t believe you aren’t here anymore. I really can’t wrap my head around it. We talked about this. So many times.
“I’m so mad, I’m so sad I don’t know what to do. You were my dearest friend. For so long. Above anything else.
“I’m so sorry I couldn’t fix or take your pain away. I really wanted to. The kindest, sweetest soul with demons he never deserved. I hope you’re okay now. rest.”
Ariana and Mac began dating in August 2016, shortly after which she was featured on his track ‘My Favorite Part’.
He also contributed to an official remix of her song ‘Into You’, while the two had also worked together on the song ‘My Way’ years before getting together, which they performed at the One Love Manchester benefit concert last year.
They split in May this year, with Ariana now engaged to comedian Pete Davidson.
In the hours after Mac’s death was first reported, it emerged that comments had been disabled on Grande’s Instagram account, as she had been receiving abusive and upsetting messages from his fans.
Mac had a long history with substance abuse and spoke openly about his issues on multiple occasions. In May, he was arrested for driving under the influence (DUI), after totalling his car and fleeing the scene.
He previously overcame an addiction to promethazine in 2012 and talked about his drug use in a 12-minute documentary called ‘Stopped Making Excuses’, which came out in 2016.