Feinberg-Mngomezulu captains Stormers into Champions Cup last 16
Feinberg-Mngomezulu, 23, slotted two conversions and a penalty as his side crossed five times in total in rainy conditions in Cape Town as they bounced back from last Sunday's 61-10 thumping at Harlequins.
Defeat left the Tigers' last 16 chances hanging by a thread, with La Rochelle needing only a single point form their home match with Quins to snatch the last knock-out spot from Pool 3.
Four-time winners Leinster produced a solid comeback to beat Bayonne 22-13 having already made sure of their knock-out berth from the group.
The Stormers named playmaker Feinberg-Mngomezulu as captain with regular skipper Salmaan Moerat ruled out due to a family engagement, while backs Warrick Gelant and Damian Willemse were both missing due to the head injury protocol.
Gelant and Williams started last Sunday's hammering at Harlequins which left the South Africans needing at least a draw against the Tigers to advance.
The visitors left England full-back Freddie Steward back home in a much-changed outfit, despite needing a win to ensure a spot in the last 16.
Later, in Pool 1, Antoine Dupont's Toulouse host Sale needing a win to reach the next round.
If the record six-time winners beat their English visitors then South Africans Sharks will be eliminated, despite hammering Clermont 50-12 earlier in the day.
In-form Glasgow host three-time champions Saracens on Sunday in the final game in the group with the Scottish side eyeing a top seed in the last 16.
In Pool 2, Toulon head to Gloucester and Munster host Castres later on Saturday with all four sides able to join Bath in the next round.
The English champions hammered Edinburgh 63-10 on Friday with the Scottish club waiting on the other outcomes to see if they make it through.
On Friday, the Bulls edged Pau 26-24 in Pool 4 to keep their hopes of progress alive with the Scarlets, who head to Northampton on Sunday, the only side who can deny them a spot in the knockouts.
Champions Bordeaux-Begles head to Bristol earlier on Sunday, with both teams already in the next round.