The 6'9" 24-year-old grabbed the 20th win of his professional career with a gritty display against the well-travelled American, who himself has fell to just his third defeat in 32 bouts.
Fury was given the nod by all three judges with scores of 119-110, 119-110 and 119-108 at the Odyssey Arena as he continues his path towards earning a shot at the world heavyweight title.
It was an easy victory for the young Fury and a largely dull fight as Johnson defended and threw few punches but took everything his opponent gave him.
Fury lacked his usual fireworks and his trademark winning combinations were conspicuously absent, apart from a couple of assured displays in the eighth round.
But the WBO Intercontinental champion started on the front foot and stayed there as he clocked up the rounds and ultimately did all the work.
Even though one-time world title challenger Johnson looked to be cooking up counters to his heavy head shots none came to fruition.
Perhaps out of frustration, Fury was deducted a point in the seventh for pushing at the break.
"Credit to Kevin - he came over here. I didn't go in there to knock him out, because I knew he was a world-class fighter who makes you miss," said Fury.
"I didn't let my heart get to me, I knew that would be a war and I outboxed him."
Fury's victory in Belfast - his third of 2012 - means he is one fight away from becoming mandatory challenger for the WBC heavyweight title, currently held by Vitali Klitschko.