The victory was the Lions first in eight Tests and condemned South Africa to only their third defeat at this ground in 15 years.
In a series which has gripped rugby fans, the Boks took to the field wearing white arm bands emblazoned with the words 'Bakkies' and 'Justice.' The towering lock was banned for two weeks in the aftermath of last week's second Test but even if he was playing this afternoon, it's doubtful South Africa would have won - particularly as De Villiers deemed it necessary to make 10 changes to the side which won the series last weekend.
Riki Flutey and Williams gave the Lions a real cutting edge in the opening exchanges while Phil Vickery turned back the clock when he destroyed Tendai 'Beast' Mtawarira at a scrum to set up Stephen Jones for the first points of the match.
They were quickly wiped out when Jamie Heaslip was penalised to give Morné Steyn the chance to level the score, which he did with ease.
Williams scored his first try after Heaslip drew Zane Kirchner beautifuly before popping inside to the Welsh winger for the opening score under the posts.
Not satisfied with scoring his first try of the tour, Williams doubled his tally after Flutey worked some magic to send Williams clear.
Simon Shaw was given the yellow card in the dying stages of the first half in a fractious affair after he put a knee to the upper back of Fourie du Preez.
Steyn banged over his second penalty on half-time to make it 15-6, a deserved lead for the tourists. The further the game went on, the more the South Africans seemed more intent on taking players out without the ball. They were scarcely hanging on when the decisive moment came on 55 minutes.
Ugo Monye came in off his wing to pluck the ball from the air to race 80m clear to the tryline. Steyn and Jones traded penalties before the Welsh pivot added another late on to ensure the Lions won a final Test match for the first time in 20 years.
Scorers: South Africa: Pens: Steyn 3
Lions: Tries: S.Williams 2, Monye; Cons: Jones 2; Pens: Jones 3