I have a green card that expired in 2019, which grants permanent residence and the ability to work. I also have an SS number. I am a British citizen, 18 years old and when applied for our cards, I was a dependent of my mother's work visa here. Now we have separate green cards. I'm not sure what particular card this is, though it says I-551 on the back.
My question is whether my card would still be valid if my mother were to leave the US back for England. Could I stay? And if not, what would be the most convenient way of being able to. Thank you.

Your permanent residence is separate and distinct from the principal. If you stay or go, you need to maintain your LPR status.