Miss Hungary, Edina Kulcsar, was judged the runner-up and Miss United States, Elizabeth Safrit, came third in the 64th annual competition, contested by women from 121 countries.
Medical student Strauss clasped her hands together in surprise and was crowned by the outgoing Miss World, Megan Young of the Philippines, to huge cheers in the ExCeL exhibition centre.
“South Africa this is for you,” Strauss said. “I think I will brace myself for what’s about to happen. It’s a huge responsibility.”
She told AFP she wanted to help spread education in her homeland and thereby help other people realise their dreams.
“I am what I am today because of the opportunities I have received and I would love to give others the same opportunities; educating, the opportunity to be educated, to make healthy choices and also to live their dreams,” Strauss said.
And she was already getting used to the glittering crown.
“I’ll definitely keep it on as long as I can but my parents are staying around here so they’ll be hugging me and I’m afraid that it might fall off,” she said.
This year’s event has been rocked by the murder of Miss Honduras, 19-year-old Maria Jose Alvarado, who was shot dead along with her sister last month.
Police in Honduras accuse her sister’s boyfriend of shooting them after seeing his partner dancing with another man at a party.
Strauss and other contestants will travel to the crime-plagued central American nation to build a school named in the sisters’ honour.
“We hope that it will really start to impress on the people in Honduras that the violence, and hurting people like those two innocent women… maybe they can think again before they take a gun,” Miss World chairwoman Julia Morley told AFP.
Sunday’s show started with a spectacular choreographed dance routine, with all the beauty queens on stage in costumes inspired by their homelands — including Miss Bolivia in a colourful feathered headdress.
While beauty remains central to the event, the women taking part are chiefly judged on their talents and involvement in good causes.
Miss Malaysia won the talent competition in a live sing-off with Miss Scotland.
In the Beauty with a Purpose section — the heart of Miss World — contestants present a charitable project they have been working on in their homeland.
In this section, the judges gave equal first place to Misses Brazil, Guyana, India, Indonesia and Kenya, unable to separate them.
In rounds held since the contestants arrived in London, Miss Finland topped the sporting competition; Miss United States was the multimedia winner; Miss Bosnia and Hercegovina won the top model round and Miss Sweden triumphed in beach fashion.
The event also featured a special 20-year award for 1994’s Miss World, Bollywood star Aishwarya Rai.
The 41-year-old Indian actress appeared on stage with her film star husband Abhishek Bachchan, their daughter Aaradhya and her mother.
The prize was for the “Dhoom 2” star’s charitable work with children with cleft palates since being made famous by Miss World.
“I stand here overwhelmed by this honour,” she said.
The 10 contestants who topped the final scoreboard were, in descending order, from India, South Africa, England, Guyana, Brazil, Hungary, Kenya, United States, Mexico and Australia.
hey were joined by Miss Thailand, who won the people’s choice award, voted for by people around the world through a mobile phone app.
The judges chose Miss Hungary, Miss Australia, Miss South Africa, Miss United States and Miss England as their top five before picking their top three.
Miss World began in 1951, a year before its chief rival Miss Universe.
Strauss is the third South African Miss World, the country having held the title in 1958 and 1974.
Venezuela tops the all-time leader board with six titles, ahead of India and the United Kingdom on five titles each.