From there, “Justin kind of kidnapped me,” Poo Bear said. Together they traveled around the world on tour — Boston, Toronto, Tampa, a break for beaches and Segwaying in Bali, Bangkok, Brazil — while working on what would become “Journals,” an underrated, under-promoted digital singles compilation, released at the end of 2013, that was crucial in introducing Mr. Bieber to new audiences and allowing him to flex some muscles of self-determination. “Before that, he was just told to sing,” Poo Bear said.

The partnership also offered Poo Bear a chance to break through on an almost unimaginable scale — if he could stick along for the ride. Mr. Bieber was “very overprotective and possessive of me,” recalled Poo Bear, who was not being paid up front for those early studio sessions. “He said, ‘There’s no need for you to work with anybody else because I’m the biggest star in the world.’ And I’d be like, ‘You know what, you’re right.’ ”

Like Mr. Bieber, Poo Bear entered the music industry at an early age. He was raised in Atlanta, where he moved at 9 with his mother and brother after his parents divorced and a tornado destroyed the family’s Connecticut home. Down South, he followed in the footsteps of the era’s local kids’ groups, signing his first record deal at 13 and another at 15.

Through a cousin in the business, he met 112 and scored his first hit with that Puff Daddy-backed group while still in high school. But Poo Bear said he was routinely the victim of bad contracts, shady lawyers and credit-hogging artists, which includes the time he spent working with the white-hot (and then burned-out) producer Scott Storch, with whom he lived in a Miami mansion for three years during that beatmaker’s mid-’00s peak.

Life with Mr. Bieber was comparatively tame and more reliable, despite the bumps, which were always well documented by the celebrity press. “I was there when he was supposedly drag racing in Miami,” Poo Bear said of a January 2014 incident involving a yellow Lamborghini that ended in a plea deal. “I was in an Escalade with his father next to him. We were going about 28 in a 35.”