The Hartlepool middleweight beat Russia's world number two Nadezda Torlopova 18-10 to book her place in the final and join Nicola Adams and Natasha Jonas in the London 2012 team.
Marshall began strongly and never looked back, managing to fight off fatigue towards the end to claim the biggest win of her career so far.
'To know that I had to win my semi-final against the world number two just to secure a place in London was a lot of pressure, but I kept my nerve and I am incredibly relieved to have finally qualified,' she said afterwards.
'It's what I have been working towards for the last three years and to know I have achieved it is amazing.
'Now I can go into Saturday's final feeling incredibly relaxed and knowing I have the opportunity to become world champion.'
Flyweight Adams secured her place in a third consecutive world championship final with a resounding 11-6 win over Russia's Elena Savelyeva.
In the final she will now face China's world number one Ren Cancan, who beat her in the final two years ago, but whom Adams has since beaten.
Lightweight Jonas had to settle for a bronze medal, though, after losing her semi-final 18-10 to another Russian, Sofya Ochigava.
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