The Kelvin Hall in Glasgow was rocking as the local boy won each of the five rounds with some clever boxing and impressive footwork, nipping in and landing his shots before getting out of the way of Williams' reach advantage.
Hyslop, who has a background in boxing, scored points with lightning fast counter punches and avoided the few takedown attempts made by Williams.
Hyslop's Dinky Ninja Fight Team (DNFT) team-mate, James Doolan, lost a unanimous decision in the other title fight at Vision 3: Uprising.
Swede David Haggstrom took the bantamweight title after five hard-fought rounds in which Doolan looked to have done enough to win rounds one and two but could not get the better of his opponent in the last 15 minutes.
Haggstrom's victory is expected to propel him into the European top 10 rankings while Doolan will look for a win in his next fight to keep from dropping out of the top 10 himself.
Stirling's Adam 'Handsome' Stevenson earned himself a shot at the Vision lightweight title after a stunning submission victory over DNFT representative Alan McCauley.
The two put on a thrilling contest which appeared to be going in Stevenson's favour, but when the chance came to finish he locked on an armbar and forced McCauley to tap in the third round.
It was Stevenson's fourth successive victory by armbar, a feat made all the more impressive by the fact he is best known for his boxing ability.
The Scottish Martial Arts Centre fighter will now take on the winner of Saturday's title fight between Bobby McVitie and Jani Lax, which takes place in Sweden at Vision Fight Night.
He put his recent winning streak down to a switch in training camp and a new attitude.
He said: "It was a move of gym, a move of training partners and a change of attitude. All those things put together results in a better Adam.
"I'm over the moon. I was confident I was sneaking ahead on the scorecards but I didn't feel it was by much.
"It wasn't the busiest of fights because I think he was a bit cautious of my power and I was cautious of his reach and fast hands.
"The plan was to soften him up with elbows and hammer fists and to slide my body into position to go for the armbar."
John Cullen and Souk Kampasath battled to a unanimous draw while Rab Whiteford's systematic dismantling of Antanas Jazbutis earned him a decision victory.
The undercard, made up of C-class fights, was topped by a back and forward contest between Carson Riches and Olly Georgopolous, which Riches won by TKO in the third round.
And in the night's only B-class bout, East Kilbride MMA's David Hamilton secured a slick submission win over Paul Patrick thanks to a first round guillotine choke.
Welterweight: Graham Black defeated Jonny Brown by unanimous decision
Lightweight: Matty Laurenson defeated Jerry Hopkins by unanimous decision
63kg Catchweight: Cammy Scott defeated Peter McCafferty by unanimous decision
Featherweight: Ross Stewart v Darren Kinloch scored as a majority draw
Welterweight: Scott Mulgrew defeated Chris Bungard by majority decision
Lightweight: Kev Devine defeated William McCurdy by majority decision
88kg Catchweight: Carson Riches defeated Olly Georgopolous by third round TKO
Welterweight: David Hamilton defeated Paul Patrick by first round submission (guillotine choke)
Flyweight: Josh Foster defeated Brian Gallacher by split decision
Featherweight: Rab Whiteford defeated Antanas Jazbutis by unanimous decision
Lightweight: Phil Flynn defeated Kieran Malone by first round knockout
63.5kg Catchweight: John Cullen v Souk Kampasath was scored a unanimous draw
Lightweight title eliminator: Adam Stevenson defeated Alan McCauley by third round submission (armbar)
Flyweight title fight: Brian Hyslop defeated Dec Williams by unanimous decision
Bantamweight title fight: David Haggstrom defeated James Doolan by unanimous decision