Millie, from Walsall, said she was a typical 21-year-old before she had the chance to save her poorly mum.
Mandy, 49, had been suffering from kidney failure for more than a year after her body's immune system began attacking her kidneys.
Millie volunteered to help after helplessly watching her become dependent upon dialysis and on Valentine's Day the mum and daughter went under the knife.
Millie said: "I nominated myself straight away - I hated seeing my mum so ill.
"It was scary but all worthwhile when I woke up and realised I had saved someone's life. Knowing it's my mum makes it feel even better.
"Being wheeled down the theatre when there is technically nothing wrong with you was the strangest situation to be in, as normally you're going in to improve your own health."
After just a week in University Hospital Coventry, Millie is now back at home and recovering.
She said: "I'm recovering really well and my mum came home yesterday too. My mum being ill has changed my life forever, I used to be a typical 21-year-old and went out all the time with friends, but now I'd rather spend time with my mum."
Ten months ago the family were told that a live donor was possible and that Millie was a match, but it took until this month for the operation to go ahead because doctors needed to check the pair were both healthy enough for the operation.
Millie said: "When we were told it was possible for my mum to have a live donor, I didn't give it a second thought to give my kidney away. I don't need two of them anyway."
Mandy suffers from a rare immune condition which is thought to occur after pregnancy. It develops when some cells from the baby remain inside the mother and her body starts to attack them. Millie is now keen to raise awareness of live donations.
he has set up a Facebook page - Our Transplant Journey Millie & Mandy - which has received nearly 3,000 likes.
Her dad, Roy, said: "I'm just so proud of Millie, it's amazing what she's been able to do for her mum."
Millie said: "It was a scary journey to go through but it was all worth it when I saw how much better she already looks.
"When we first saw each other after the operation we both cried, it's such an emotional thing to go through but I'd do it all over again for my mum."