Watson was born in Mexborough, Yorkshire, the son of a headmaster in a mining community. As a child, Watson spent time on his Uncle George's farm. The slaughtering of a pig on the farm horrified Watson; he said his view of farm life changed from idyllic to a death row for animals. Watson began to reassess his practice of eating meat. He became a vegetarian in 1924 at the age of fourteen, making a New Year's resolution to never again eat meat. He gave up dairy products about 18 years later, having understood the production of milk-related products was also unethical.
He said: "I was surrounded by interesting animals. They all "gave" something: the farm horse pulled the plough, the lighter horse pulled the trap, the cows "gave" milk, the hens "gave" eggs and the cockerel was a useful "alarm clock" - I didn't realise at that time that he had another function, too. The sheep "gave" wool. I could never understand what the pigs "gave", but they seemed such friendly creatures - always glad to see me."