The 16-year-old set new personal best scores for both the free skate (160.78) and overall score (245.09).
It caps a stellar season for the Russian, who won two Junior Grands Prix, a Challenger Series senior event, and finished second at both the Junior Grand Prix Final and Lausanne 2020 Winter Youth Olympics.
Youth Olympic champion Kagiyama Yuma struggled with two big errors in his free skate, and could only finish second with 231.75 points.
Mozalev's teammate Petr Gumennik (231.12) finished third after the USA's Andrew Torgashev, third after the short program, fell four times during his free skate.
Kagiyama stutters in free skate
Kagiyama, the leader after Wednesday's short program, was drawn to skate before Mozalev in the free program.
Things did not get off to a good start for the Japanese, falling on an under-rotated quadruple toe loop that was meant to be part of an opening combination.
He bounced back with solid jumping passes, including a triple loop, and another quadruple toe loop in combination with a double toe loop, although the triple Axel in his next combination was marked under-rotated too.
But he lost more points in base value when he was only graded level threes on his Flying Camel Spin and step sequence, then popped his closing triple Axel down to a single. He was also called for an unclear edge on his triple Lutz.
A visibly disappointed Kagiyama scored 145.93 points for his free skate, only good for fifth in the segment, and a total of 231.75 that put him only 0.63 ahead of Gumennik who had skated previously.
Watch Kagiyama Yuma's gold medal-winning performance from the Lausanne 2020 Winter Youth Olympic Games below.
Mozalev stays calm
Skating after his Japanese rival, Mozalev knew the score he needed to beat and was able to put in a calm and collected skate.
He opened with a quad toe, double toe combination and followed that with another quad toe and then a triple loop to give himself breathing room in the scores.
The Russian's only error came on his triple Axel, double toe loop combination when he put his hand down on landing on the Axel.
But the second half of his free skate, with the three jump elements worth 1.1 times their normal base value, was marked as clean.
Mozalev and his coaches were up on their feet in the kiss and cry upon seeing the scores come in, taking a bow before the camera.