Crossbreed Bill was found in an exposed porch in the cold and lying on a wet blanket outside a property in Hull in February after a member of the public alerted the charity.
He had been left out in the cold for two days because "his owner thought he had already died".
Inspector Laura Barber said she had "no idea how he survived," but added: "I am so pleased he has".
Talking of the rescue, the inspector said Bill had wagged his tail as she approached, although she could see that he was very weak.
"He was thin and had a huge, open sore on his back," she said.
The RSPCA Scarborough and district branch took in Bill as he needed emergency surgery to remove the lump on his back, as well as neutering and a dental procedure.
Bill underwent a series of operations to have tumours removed from his face and body and had to have his spleen removed.
He has now been rehomed with a family from Acomb, in York.
Bill was adopted by Fiona Cunniff and her husband Paul and daugher Amelie, 16, after undergoing months of rehabilitation at the York, Harrogate and district branch of the RSPCA.
Mrs Cunniff said they spotted Bill on the York RSPCA website whilst looking for a younger dog but, "felt a connection".
“We have a Jack Russell dog called Nelly who is 15 and we thought it would be nice to have a friend to help keep her young," she said.
“We went to see him with Nelly and he was so lovely and got on with us all so we were smitten."
As for Bill enjoying his new family home, Mrs Cunniff said: “We thought with him being an older dog he would just plod about and sleep a lot but he has so much more energy and enthusiasm for life now - he is living it to the full and he absolutely loves his walks.”