He first met his now "forever" family when he was 18 months old.
The new mother added that he felt like a part of the family since the beginning of their time together, calling him her "valentine baby", ABC notes.
Young Michael was in foster care for 832 days, ABC News reports.
A tweet with Michael raising his fist in triumph and posing with the family has gone viral - with over 50,000 retweets.
The boy, Michael, was adopted Tuesday morning in Phoenix. "His original case plan was reunification with his 'bio-mom.' When that plan failed, a new plan was set".
Montgomery said that, as a single parent, she was not initially looking to adopt Michael, just to help him.
Goodwill gesture: Pakistan frees 220 Indian fishermen
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One of his new adoptive sisters, Dezhianna Brown, shared a picture of Michael at the courthouse on Twitter, and it quickly went viral.
In her tweet, which features a video of Michael smiling and running around, Jordae simply wrote: "Officially adopted".
"He is the most loving, smart, amusing and happy kiddo", Ms Montgomery said.
"My children's last name is Brown his was already Brown when he got here so it was again meant to be", Montgomery said. "He is so happy, always laughing", Montgomery said.
Montgomery indicated that a photo showing Michael holding a chalkboard reading, "Some things are worth the wait", perfectly summarizes his personality. "He couldn't help but let everyone know that he was being adopted today".
We learned about Michael through social media, all thanks to his 17-year-old sister Dea.