Middleweight Marshall beat Azerbaijan's Elena Vystropova in Quinhuangdao to upgrade the silver she won two years ago and give herself the perfect gift on her 21st birthday.
Marshall will now look to win another gold when women's boxing makes its Olympic debut at London 2012 - where she will lead a full complement of three British boxers all with medal potential.
Lightweight Natasha Jonas won bronze in China while European champion Nicola Adams claimed flyweight silver, beaten 14-10 by home favourite Ren Cancan in her final.
"It’s been an amazing week and to come away as world champion and with a place at the Olympics is pretty incredible," said Marshall.
"It’s great to go one better than the last time and now I am just looking forward to the Olympics and making sure I am in the best possible shape to be successful again in London."
Adams was also quick to reset her sights on the summer after her narrow final defeat.
"I wanted the gold and sadly it was not to be, though overall I have to be happy with this week as I have reached a third World Championship final and guaranteed my place at London," she said.
"The Olympic Games is the big prize and I hope to go one step further in London."
Great Britain will take ten boxers to the Olympics, seven men and three women - and will only not contest three of the weight divisions.
"This is a great end to a superb week for amateur boxing in Great Britain that has seen the whole team box extremely well," said team performance director Rob McCracken.
"Savannah has performed superbly throughout the competition and to come away as world champion from such a tough draw in a high-class field is an indication of her immense talent.
"She is still very young yet this is her second appearance in a World Championship final and, as long as she continues to work hard, Savannah can achieve anything she wants in this sport.
"Since we started the women’s programme in 2010, all of the squad have worked incredibly hard to get to where they are today.
"Their achievements are a well deserved reward for their dedication and commitment and a fitting tribute to the superb efforts of the all the coaches and back-up team that have supported them every step of the way."