After the Proteas had won the toss and elected to field, new-ball pair Marizanne Kapp and Shabnim Ismail started superbly to reduce England to 23-3 in the fourth over.
Ismail took the wickets of Amy Jones and Alice Capsey while Kapp dismissed Danni Wyatt-Hodge to give the South Africans a really good start to the contest.
England though fought back led by their captain Nat Sciver-Brunt, returning from a calf injury, and former skipper Heather Knight as the experienced consolidated before launching as the overs counted down.
The pair put on 133 for the fourth wicket with Nat Sciver-Brunt striking 75 from 47 balls while Knight contributed 58 from the same amount of deliveries that took the home team to a total of 169-5.
Proteas captain Laura Wolvaardt spoke before the game about looking to get back to her best with the bat, and made her way to 17 before being caught at mid-off in what was a huge wicket for England.
Fellow opener Tazmin Brits scored 51 but received no support as wickets continued to fall at regular intervals as the Proteas failed to keep up with the required rate.
The South Africans eventually only reached 129-8 as England prevailed and will now meet arch-rivals Australia in the final at Lord's on Sunday.
The South African Women were made to work hard for runs as they eventually chased down 117 to defeat Bangladesh by four wickets in the Women's T20 World Cup at Lord's on Sunday.
Pakistan finished their ICC Women's T20 World Cup campaign on a high after beating Netherlands while Ireland shocked the 2016 champions West Indies in Bristol.
The durable Marizanne Kapp was instrumental in the Proteas Women's victory over India in their must-win Women's T20 World Cup clash last Sunday.
Australia Women star Ellyse Perry wrote another chapter in her storied career by almost single-handedly guiding her side to an 113-run win over Pakistan at the ICC Women's T20 World Cup.
