Chloe Allen joined the Scots Guards as a man in 2012 but began hormone therapy in the last month, and has officially changed her name.
"All the paperwork within the Army, within the battalion, has been changed and sorted out. My passport will be done shortly," she told BBC.
"It's brilliant, it's amazing... To say everything that I've wanted it to say and for me to still be serving as an infantryman is even better," she added.
The application process for female recruits was due to open later this year after the UK government had opened up frontline roles to women.
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Guardsman Allen had initially thought she would have to leave her post in the First Battalion Scots Guards.
But she decided to talk to a careers officer and was informed she could continue in her role as a rifleman and driver of a Mastiff armoured truck.
General Sir James Everard, Commander of the Field Army, said: "I'm delighted to have our first woman serving in a ground close combat unit.
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