Wales Under-20s star Williams, who arrived in the summer, gave the Blues the lead as they made a flying start in Treviso but they found themselves trailing for much of the encounter.
The Italians bagged three first-half tries and sealed the bonus-point after the break but the boot of Steve Shingler had kept Danny Wilson’s side in touch.
Macauley Cook sparked the comeback following some fine work from Halaholo and the Super Rugby-winning centre soon turned from provider to scorer.
The victory put a halt to a three match losing streak in the Guinness PRO12 and the region now turn their attention to the Anglo-Welsh Cup.
The Blues made an assured start at Stadio Monigo with Matthew Morgan particularly lively and Steve Shingler pulling the strings.
They claimed the first try on six minutes after Morgan’s third counter attack of the evening made major inroads. Three phases later the ball was worked left and debutant Williams skirted round the outside before stepping the full-back and easing under posts.
Shingler converted the effort to give his side an early 7-0 lead however it was short lived as Treviso hit back from their first real attack.
It came in fortuitous circumstances as both teams failed to claim a Shingler up-and-under. Willis Halaholo appeared to be pulled back but recovered to make the try-saving tackle however the Italians were camped on the Blues line and Dean Budd eventually barged over.
Ian McKinley slotted the conversation to draw level and they soon took the lead with a fine try.
A series of short passes and offloads unlocked the Blues defence and McKinley sliced through. He then drew Morgan to put captain Alberto Scarbi away and then added the extras.
Shingler cut the deficit with a pair of penalties on 21 and 26 minutes as Wales’ Capital Region regathered their composure.
However they conceded a third try on 32 minutes as a clever kick from McKinley put the pressure on the visitors.
They initially survived but Treviso came back and following a sustained period of pressure the Blues ran out of numbers and Michael Tagcakibau dived into the corner with McKinley adding the extras.
Shingler kept his side in touch, slotting his third penalty to bring the Blues within five points at the interval.
However they were dealt a further blow five minutes into the second-half as Treviso claimed a bonus-point try.
They made a flying start to the half and were held up over the line in the opening minutes and following a succession of phases McKinley dinked a cross-kick into the corner with Abraham Steyn collecting unopposed.
McKinley maintained his 100 per cent strike rate with the conversion to make it 28-16, it left the visitors facing an uphill task but they responded almost immediately with a pair of fine attacks.
First Rey Lee-Lo and Williams combined to release Lloyd Williams. He was hauled down but from the next phase Willis Halaholo sucked in two defenders before moving the ball to Ellis Jenkins who offloaded magnificently off the floor to put Cook away.
Shingler was unable to bisect the posts but Wilson’s side were now within seven points and he soon added another penalty.
The fly-half was off target with a long-range shot on the hour mark but Cardiff Blues were building momentum.
They continued to battle and finally wrestled back the lead as Halaholo jinked past two defenders to claim a vital try on his first appearance for the region.
Shingler converted expertly from the touchline to make it 31-28 to the visitors but alarm bells were almost immediately ringing after Lee-Lo lost the ball following the restart.
It appeared ominous but Williams came off his wing to make a vital tackle and then held the player up to earn a scrum.
The Blues continued to battle and Shingler added a late penalty to stretch his side’s lead to six points and they clung on for the vital victory.