In January 1914, he enlisted in the army, and after war was declared, he was mobilised on August 5. The Christmas truce of 1914 impacted him greatly. He became disgusted with the pointlessness of the war and was angry at the greed and bigotry he saw as causing it. He became determined that Germany and Britain should never go to war again.
He told of his war experiences in The Wet Flanders Plain (1929), The Patriot's Progress (1930) and in many of his books in the semi autobiographical 15 book series A Chronicle of Ancient Sunlight (1951-1969).
After the war, he read Richard Jefferies' book The Story of My Heart. This inspired him to begin writing seriously. In 1921, he moved to Georgeham, Devon, living in a small cottage. He married Ida Loetitia Hibbert in 1925. Together they had six children.
In 1927, Williamson published his most acclaimed book, Tarka the Otter. The book won the Hawthornden Prize. It also sparked a long running friendship with T. E. Lawrence.
In 1935 Henry Williamson visited the National Socialist Congress at Nuremberg and was greatly impressed, particularly with the Hitler Youth movement. In England he joined Oswald Mosley's British Union of Fascists.
In 1936 he bought a farm in Norfolk and became a farmer. The Story of a Norfolk Farm is based on his experiences.
At the start of World War II, Williamson was briefly interned for his well known political views but was released after a weekend. After the war, the family left the farm. In 1946, Williamson went to live in Devon alone and in 1947 Henry and Loetitia divorced.
Williamson fell in love with a young teacher, Christine Duffield and they were married in 1949. He began to write his great series of fifteen novels collectively known as A Chronicle of Ancient Sunlight. From 1951-1969 he produced almost one novel a year. This put great strain on his marriage and in 1968 they were divorced after years of separation.
In 1974, he began working on a script for a film treatment of Tarka the Otter but it was not regarded as suitable to film. Filming for the movie went on unknown to him. The movie, narrated by Peter Ustinov was released in 1979. On his eightieth birthday he hoped for some honour from the British government. Bitterly disappointed, his health began failing rapidly. Suffering from senile dementia, he died on August 13, 1977 and was buried in the churchyard of Georgeham. In 1980, the Henry Williamson Society was founded.
All dates are for when first published