Dawid Malan and captain Jos Buttler hit centuries to set up a 59-run win for England in the third and final one-day international against South Africa at the Diamond Oval in Kimberley on Wednesday.
Malan made 118 and Buttler scored 131 in an England total of 346 for seven. South Africa were bowled out for 287 with fast bowler Jofra Archer taking a career-best six for 40.
South Africa had already clinched the series by winning the first two matches but England captain Jos Buttler said Wednesday's win was a reward for the hard work his players had put in during the short series.
"There's been some really good stuff. Guys getting hundreds and Jofra Archer was brilliant. We've got a good thing going. We're building nicely," said Buttler, who was named player of the match and player of the series.
South African captain Temba Bavuma said the series win was important. "We came in under a lot of pressure. We've done ourselves justice and come closer to automatic qualification for the World Cup."
South Africa need one win in their remaining two matches against the Netherlands to move ahead of West Indies into one of the eight qualifying places in the World Cup Super League, although Sri Lanka could still move ahead of them in a forthcoming series against New Zealand.
The left-handed Malan and his right-handed captain shared a fourth wicket partnership of 232 off 211 balls after coming together with their team in trouble at 14 for three in the sixth over.
A slow start to the partnership became a torrent of runs as they took command on a slow pitch.
England scored 217 runs in the last 20 overs of their innings, scoring rapidly even after the two centurions were dismissed.
South Africa opted to go into the match without their two fastest bowlers, Anrich Nortje and Kagiso Rabada, and appeared powerless to keep the scoring in check.
When South Africa batted they were 59 runs ahead on comparative scores after 30 overs. The difference was that England had more strike power in the middle overs, with Archer and leg-spinner Adil Rashid (three for 68) taking crucial wickets.
Archer, in the second match of an international comeback after a long injury-enforced lay-off, bowled consistently at speeds of more than 145km/h (90mph).