'The Punisher' - a former Deanery High School and St John Rigby student - enjoyed a unanimous 30-27 points victory at the O2 Arena.
Mokaev had seen off Cody Durden inside a minute of his UFC debut in London back in March.
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This time he was taken the distance by the durable Johnson - who came into the contest with 10 wins from his 12 bouts - but extended his pro record to 7-0, after a perfect amateur career across 22 fights.
Mokaev was born in Dagestan but arrived in Wigan with his father as a refugee in 2012, shortly after the death of his mum.
He learned to box and wrestle at Wigan Youth Zone before making his amateur debut in 2015 in Liverpool, and now trains in Salford.