Sophie Fearns said son Charlie, of Merseyside, needs to fly back to Seattle, America for experimental treatment unavailable in the UK.
She said he urgently needs more T-Cell Therapy at Seattle Children's Hospital after the last transplant failed.
His mother said she was "ecstatic" and "overwhelmed" by the "selfless amazing person" who paid for his treatment.
Charlie has been ill since the age of three, she said.
After a third relapse in April she said there was "no option" left for treatment for him in the UK.
Mrs Fearns said the family, of Litherland, were left "heartbroken" when the first T-Cell transplant did not work.
She said the news the treatment has been paid for was "completely overwhelming".
"We don't know who this donor is but from the bottom of our hearts it means an incredible amount - not just from the foundation but everybody that supports Charlie."
Charlie's father Rob Fearns said they were "extremely grateful" for the generosity of the anonymous donation.
"He or she specifically asked to pay for Charlie.
"We thought we were going to have to fight tooth and nail like last time to get the funds together."
His parents said they received the confirmation via an email from the hospital on Wednesday saying someone had offered to foot the bill for them.
Now Charlie has had financial clearance from the hospital, the treatment will start as soon as possible.
Mr Fearns said he is expecting to find out the date next week.
Information from Seattle Children's Hospital
A mystery donor has paid Â£150,000 to fund a "last chance cure" for a boy of 11 with leukaemia, his mother has said.