The winger's converted score, with four minutes to go, topped off a spirited second half fightback from Treviso who had scrum half Tobie Botes to thank for 21 of their points.
South African Botes scored a try, two conversions and four penalties, with Chris Burton (drop goal) and Ezio Galon (try) also making scoring contributions.
The Stadio di Monigo faithful roared their approval when Botes kicked the hosts ahead from a third minute penalty, but the Scarlets, captained by new Wales cap Rob McCusker, went on to dominate the opening half.
Fly-half Rhys Priestland took his pre-season form into the competitive season, attacking with vigour and inspiring his team-mates to take the game to Treviso.
Priestland used quick ball from a lineout, outside the hosts' 22, to ghost past two tacklers and dummy his way around full-back Luke McLean for a very well-taken try, which he converted himself.
Treviso number 10 Burton replied with a lovely drop goal from 40 metres, but the Scarlets' tails were up and just two minutes later, teenage winger George North scored a try on his competitive debut.
Countering after Treviso had put boot to ball, winger Andy Fenby and full-back Morgan Stoddart traded passes before centre Gareth Maule's inside delivery gave North the opportunity to test the hosts out wide, and he impressively beat two men for the corner.
Priestland was wide with his conversion attempt and although Botes managed to reduce the arrears to 12-9, Nigel Davies' men were soon back on the try-scoring trail. Scrum half Martin Roberts had the ball knocked from his grasp as he crossed the whitewash.
A Priestland penalty stretched the lead to six points and Treviso were reduced to 14 men when impressive number 8 Manoa Vosawai saw yellow for coming in at the side of a ruck.
The Scarlets picked up another seven-pointer before the break, with centre Regan King involved in the build-up. His inside pass sent North, the North Walian prospect, powering over from close range to complete his brace.
Yet, Treviso hit their stride in the third quarter to really make a game of it, showing great commitment to the cause in front of a hugely vocal crowd.
The Scarlets failed to build on some early territory, and Botes punished them with a fine penalty and try, the latter coming after a concerted spell of pressure from the Italians which included an inspirational run from McLean.
The sight of Italian international centre Galon dashing over close to the posts for another try had Treviso sensing an upset, and Botes' conversion pushed them in front at 24-22.
The try was the result of another sweeping move and Benvenuti did really well to pop the ball back inside to Galon as North tackled him towards touch.
Both coaches looked to their benches for some inspiration in a strength-sapping second half, and the Scarlets needed to regain their composure and discipline as Botes extended the lead to five points.
A big hit from Jonny Fa'amatuainu brought the Scarlets back to life and Priestland kicked two penalties in a five-minute spell to see his side regain the lead.
A late defensive lapse from the visitors undid all their good work, however, as Benvenuti had enough space out wide to cross in the corner. Botes added the extras for good measure from the touchline, and tenacious Treviso hung on for a memorable opening win.
The result is a huge fillip for Smith's charges, who had some terrific displays from the forwards, including Italian internationals Leonardo Ghiraldini, Alessandro Zanni and Vosawai. They will travel to title holders, the Ospreys, next week with renewed confidence.
|Benetton Treviso Score Card|
|Scarlets Score Card|