The pair were together for nearly two years before going their separate ways in May, after which Grande got engaged to Saturday Night Live star Pete Davidson. Despite their breakup, however, Grande appeared to always want the best for Miller, who had been open about his struggles with substance abuse and sobriety.
A source tells ET that Grande is devastated over the death of Miller. “Although the two ended their relationship, Ariana always shared a deep connection with Mac and wanted nothing but the best for him," the source says. "Ariana is very upset."
Here's what Grande and Miller have said about each other through the years.
July 2013: Grande Praises Miller as a 'Great Guy'
Months after sharing a kiss in the music video for their song together, "The Way," Grande opened up about Miller in a radio interview with Los Angeles' Power 106. "I'm gonna say a one, just 'cause of the cigarettes," she joked when ranking their kiss, but insisted the rapper was "a great, great, great guy."
September 2016: Romance Rumors to Instagram Official
Grande and Miller collaborated again three years later on her song, "Into You," and ignited romance rumors in August 2016. They attended the 2016 MTV Video Music Awards together that month, and by September, they were Instagram official. “Baabyyy," Grande captioned a photo of herself cuddling Miller, before opening up about their romance to Ellen DeGeneres.
"This is so crazy. I’ve never had the relationship talk on a show before," she said on The Ellen DeGeneres Show, blushing when the host asked if she was happy with Miller. "Yeah."
And Miller Reveals How They Became Something More
The twosome also worked together on Miller's song, "My Favorite Part," which fans guessed might be about his girlfriend. "I wrote that love song before I saw her. We’re very good friends first and foremost,” Miller told People at the time. “We made that song and started becoming close again. It’s very dope and I like how that worked out.”
He added, “We hung out for a long time and everything just happened organically. We love making music together -- we do that always. But she’s my best friend in the world.”
May 2017: The Rapper Supports Grande After Manchester Tragedy
Grande's world was rocked when 22 people were killed in a terrorist attack following her Manchester concert. Miller was by his girlfriend's side from the moment she touched down in the U.S. after the incident and joined her onstage for a duet during her One Love Manchester concert to aid those affected by the attack.
"Tonight is all about love," Grande said to the audience, looking a little bashful as her 25-year-old beau joined her onstage. The event also sparked engagement rumors, after fans noticed a piece of jewelry on Grande's left hand.
One month later, Miller couldn't help but gush over Grande in a touching post for her birthday. "Happy Birthday to this adorable pure soul who has reminded me what being happy feels like," he wrote alongside a since-deleted photo. "Thank you for loving me so good. I think it's supposed to be 'so well' but I don't care. I love you and can't wait for all of the adventures."
April 2018: Miller Welcomes the 'One of a Kind' Singer Back to Music
Miller and Grande attended Coachella together in April, where Grande performed her new single, "No Tears Left to Cry." "Very proud of this girl right here," Miller tweeted. "Welcome back. We missed you. One of a kind."
May 2018: Grande Says She Has 'Endless' Respect for Miller as They Break Up
Grande confirmed her and Miller's split just weeks later. "This is one of my best friends in the whole world and favorite people on the planet," the singer wrote on her Instagram Story in May. "I respect and adore him endlessly and am grateful to have him in my life in any form, at all times regardless of how our relationship changes or what the universe holds for each of us!"
The former Nickelodeon star concluded her message, writing, "Unconditional love is not selfish. It is wanting the best for someone even if at the moment, it's not you."
And Then She Calls Their Relationship 'Toxic'
Miller was arrested on a charge of DUI just weeks after he and Grande broke up, and when a fan tried to blame the pop star for the incident, she fought back.
Ariana Grande Opens Up About 'Toxic' Relationship With Mac Miller
"I am not a babysitter or a mother and no woman should feel that they need to be. 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*t together is a very major problem."
"Let's please stop doing that," she continued. "Of course I didn't share how hard or scary it was while it was happening but it was. I will continue to pray from the bottom of my heart that he figures it all out and that any other woman in this position does as well."
July 2018: Miller Says He's 'Happy' for Grande After She Becomes Engaged to Pete Davidson
Grande's whirlwind romance with Davidson took the world by storm, but Miller said he was "happy" for his ex. In a July interview with Zane Lowe on Beats 1 on Apple Music, the rapper reflected on their two-year romance, saying he was "in love" with the singer.
"Life is stressful," he explained. "So, of course, there were stressful times. It's not that unique. You know, like, I was in love with somebody. We were together for two years. We worked through good times, bad times, stress, and everything else. And then it came to an end and we both moved on. And it's that simple, you know."
Miller continued, adding that people had been checking up on him since his and Grande's split but insisting that he's happy they both moved on.
"It's strange. Like, the whole thing is a little strange but it's not negative," he said. "It's just a part of something that's going to continue to help make me who I am. You know, it's all positive energy. I am happy for her and moving forward with her life just as I'm sure she is with me."