Daniel Mickelson died as a result of fentanyl and cocaine toxicity, the Los Angeles County Medical Examiner-Coroner confirmed to E! News.
In July, the rising star - known for his role in the 2019 TV show Mani - died suddenly on July 4 at the age of 23, with his cause of death having remained a mystery over the last three months.
The manner of death of the promising actor, who was the older brother of model Meredith Mickelson, was ruled an accident by officials.
During the Fourth of July weekend, Meredith, 22, confirmed that her brother had sadly passed away and shared a very poignant tribute to him.
She wrote on July 5 alongside a photo of the two siblings when they were children: "[My] heart is shattered & to write this feels so wrong and i don’t even know what to say. Yesterday i lost my brother, best friend & the other half of my heart.
"There wasn’t a person i loved more on this earth. theres no words that can do him justice that i could write."
Meredith concluded the touching tribute: "To know him was to love him. he was the happiest brightest smiley most sunshine human to exist and I’m so happy God chose me to be his sister for his whole amazing life. [sic]."
In response to the upsetting news, a number of fellow celebrities took to the comments to share their devastation over the young star's death.
Model and actress Kaia Gerber posted a screenshot of a FaceTime call she had with Daniel.
She captioned the shot: "I remember that time we sat on the couch and spent the whole day coming up with our own secret language that we continued to speak every time we saw each other."
She continued: "I wish we could go back there. I wish we were still talking in sentences that probably annoyed everyone else but made us crack up every time.
"I wish I was still sitting on my bathroom floor because that was the only place I had wifi and I never wanted to miss a call from you."
"Thank you for being the reason for so much laughter and happiness in the world. It won't be the same without you here. I love you Daniel," she added.