Surgeons were able to reattach the 1-month-old's finger and he was recovering at home, Sky News reported Sunday.
Police said the fox entered family home in Bromley, southeast London, through an open back door Wednesday and found its way to the nursery, where it grabbed the boy and dragged him from his bed.
The mother said she had to "wrestle the fox off the baby," a neighbor, Paula Wellington, told Sky News.
"I've seen foxes around here before but I've never heard anything like this," Wellington said.
She said the family has since moved.
A Metropolitan Police spokesman said police were called in by hospital staff reporting about the boy being attacked by a fox.
London Mayor Boris Johnson said the problem of foxes in the city must be examined.
"Thankfully this sort of attack, though terrible, is rare, but we must do more to tackle the growing problem of urban foxes," he said. "They may appear cuddly and romantic but foxes are a pest and a menace, particularly in our cities."
A Royal Society for the Prevention of Cruelty to Animals spokeswoman told the BBC foxes usually don't attack people.
"It's not typical fox behavior at all. Foxes will come closer to a house if there are food sources," the spokeswoman said. "Then they can become quite bold, but they usually do back off and run away when there's people around."