The 20-year-old centre-back will join the Under-21s squad following recent spells in the development squad at Bournemouth and the first-team at Weymouth FC, where he made five appearances last season in National League South.
Francis began his journey at Swindon Town’s Academy and also played a season of first-team football for North Leigh FC.
Commenting on his move to south London, Francis said: "I'm buzzing to sign for this great club and I'm really looking forward to this next chapter. The hard work continues."