James Alfred Wight was born on 3 October 1916, in Sunderland, County Durham, England, to James (1890–1960) and Hannah Bell (1890–1980) Wight. Shortly after their wedding, the Wights moved from Brandling Street, Sunderland to Glasgow in Scotland, where James took work as both a ship plater and pianist for a local cinema, while Hannah was a singer, as well as a dressmaker. For Alf's birth, his mother returned to Sunderland, bringing him back to Glasgow when he was three weeks old. He attended Yoker Primary School and Hillhead High School. From his father he gained a passion for Sunderland Football Club and remained a lifelong fan. In 1992 he was named a Life President of the club.
In 1939, at the age of 23, he qualified as a veterinary surgeon with Glasgow Veterinary College. In January 1940, he took a brief job at a veterinary practice in Sunderland, but moved in July to work in a rural practice based at 23 Kirkgate in the town of Thirsk, Yorkshire, close to the Yorkshire Dales and North York Moors, where he was to remain for the rest of his life. On 5 November 1941, he married Joan Catherine Anderson Danbury, known as Helen Alderson in his books. The couple had two children, James Alexander (Jim), born 13 February 1943, who also became a vet and was a partner in the practice, and Rosemary (Rosie), born 1947, who became a physician in general practice.
Wight served in the Royal Air Force in 1942. His wife moved to her parents' house during this time and, upon being discharged from the RAF as a Leading Aircraftman, Wight joined her. They lived there until 1946, at which point they moved back to Kirkgate, staying until 1953.
Later, he moved with his wife to a house on Topcliffe Road, Thirsk, opposite the secondary school. The original practice is now a museum, "The World of James Herriot", while the Topcliffe Road house is in private ownership and not open to the public. He later moved with his family to the village of Thirlby, about four miles from Thirsk, where he resided until his death. Wight intended for years to write a book, but with most of his time consumed by veterinary practice and family, his writing ambition went nowhere. Challenged by his wife, in 1966 (at the age of 50), he began writing. After several rejected stories on other subjects like football, he turned to what he knew best. In 1969 Wight wrote If Only They Could Talk, the first of the now-famous series based on his life working as a vet and his training in the Royal Air Force during the Second World War. Owing in part to professional etiquette, which at that time frowned on veterinary surgeons and other professionals from advertising their services, he took a pen name, choosing "James Herriot" after seeing the Scottish goalkeeper Jim Herriot play for Birmingham City F.C. in a televised game against Manchester United. If Only They Could Talk was published in the United Kingdom in 1970 by Michael Joseph Ltd, but sales were slow until Thomas McCormack, of St. Martin's Press in New York City, received a copy and arranged to have the first two books published as a single volume in the United States. The resulting book, titled All Creatures Great and Small, was a huge success, spawning numerous sequels, movies and a successful television adaptation.
Wight was found to have prostate cancer in 1991, and underwent treatment in the Lambert Memorial Hospital in Thirsk. He died on 23 February 1995, aged 78, at home in Thirlby. His wife, Joan Wight née Danbury, died four years later on 14 July 1999.
On 29 July 2009, UK-based open-access rail operator Grand Central Railway, which operates train services from Wight's birthplace of Sunderland to London King's Cross (calling at Thirsk), named Class 180 DMU No. 180112 (British Rail Class 180) "James Herriot" in his honour. The ceremony was carried out jointly by Alf Wight's daughter Rosie and son Jim.
In October 2014, a statue of Wight was unveiled by actor Christopher Timothy at Thirsk Racecourse. Timothy played Herriot in the television series.