The American international scored what proved a killer try on 65 minutes when he latched onto a deft inside pass from Marcelo Bosch before slicing through the Sarries defence to touch down by the posts.
More neat work from Bosch had helped Biarritz into the lead moments earlier, his superb cross-field kick finding Imanol Harinordoquy out on the wing, who had the simple task of collecting and touching down.
The first points of a low-quality encounter did not arrive until the 51st minute through Owen Farrell's boot, however Alex Goode's last-gasp converted score at least ensured Saracens left France with what could prove a vital bonus point.
The win means Biarritz have now lost only four of 39 Heineken Cup meetings at fortress Parc des Sports Aguilera.
Charlie Hodgson got the game underway and it was Saracens, fresh from their 42-17 win over Treviso last weekend, who made the early running with numerous forays inside the Biarritz 22.
Despite Sarries' early pressure it was the home side who had the first opportunity to take the lead, but Bosch was off target with his eighth-minute penalty attempt.
Saracens continued to dominate in terms of territory and possession, although Farrell was also off-target with a penalty before Bosch missed another chance on 27 minutes.
Kelly Brown came closest to crossing the tryline in the opening period but instead of passing he opted to go alone and was held up two metres out.
Farrell finally broke the deadlock with a penalty on 51 minutes, but that score only served to spark Biarritz into life as the French outfit hit back immediately.
A swift break down the left flank put Sarries on the back foot and Bosch kicked across field superbly to France international Harinordoquy who, in acres of space, collected and crossed by the posts.
Bosch was once again off target with his place kick, however, fortunately for the Biarritz No.10, so was Farrell as his third penalty attempt bounced back off a post.
Only a brilliant last-ditch tackle from James Short prevented Ngwenya crossing in the corner but, moments later, Biarritz's pressure did tell as Jacques Burger was penalised for going over the top and Julien Peyrelongue kicked the ensuing penalty.
Saracens, now chasing the game, were exposed yet again 15 minutes from time, but this time it was down to the sheer brilliance of Ngwenya who showed a quick turn of foot to pierce the defence and touch down under the posts.
Sarries had a try chalked off on 69 minutes when Short was held up, but Goode's timely score ensured the visitors walked away with a deserved bonus point.
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