Rocco, a Rottweiler, had broken free of his leash and run in front of a car, injuring his hind leg.
PC Lou Richards found him while working her last shift in the mounted branch of the Metropolitan Police.
She managed to track down a vet who treated him free of charge, to the joy of his owner, Darren Thornton.
Three days later a box of chocolates and a Christmas card addressed to the "Three Angels", PC Richards and her fellow officers PC Claire Twig and PC Holly Hine, arrived at the police horses' stables from Mr Thornton.
PC Richards said the card and presents meant more to her "than anything else I've done in my career".
PC Cath I'Anson, of the Met's mounted branch, said Darren had become Rocco's owner after he started walking him on behalf of an elderly lady who went into a care home.
"Since then the two of them have been inseparable and it has meant a lot to Darren that our officers were able to ensure Rocco received the care he needed," she said.
"Life is pretty tough for Darren at the moment and it is really heart-warming to see local officers and residents coming together to help out."
A homeless man has bought chocolates and a Christmas card for a police officer who helped his injured dog on her last shift before retirement.