Horsell was born in Brinkworth, Wiltshire. Before the age of twenty he was preaching the gospel and became a temperance activist in 1833. In 1838, Horsell established the Anti-Nicotine Society at Congleton, Cheshire. Horsell founded the Nature's Beverage Society in 1842. The Society aimed to spread abstinence from all artificial beverages.
Horsell operated a hydropathic infirmary at Northwood Villa, Ramsgate. It has been described as the first vegetarian hospital in Britain. In 1847, a meeting was held at the hospital from which the Vegetarian Society was formed. Horsell was secretary of the Vegetarian Society for several years. In 1856, Horsell noted that there were a thousand members of the Society. He managed the Society from his London office.
Horsell edited the Truth Tester, which became the Society's official journal. The journal described vegetarianism as "the next practical moral subject which is likely to call forth the virtuous energy of society". In 1850, it was renamed the Vegetarian Advocate. Horsell stepped down as Secretary and his journal ceased in 1850. From 1849, the Vegetarian Society's President James Simpson published the Vegetarian Messenger. In 1850, Simpson moved the Vegetarian Society office to Manchester and Vegetarian Messenger became the Society's official journal. Horsell remained active with the London branch of the Vegetarian Society.
He authored a popular hydropathic manual and was an advocate of phrenology. He was a publisher for vegetarian and spiritualist literature. His wife Elizabeth Horsell was also a vegetarian.
In 1849, Horsell published Asenath Nicholson's Kitchen Philosophy for Vegetarians, in London. A review in the Vegetarian Advocate, noted that "butter and eggs are excluded" from the recipes. The Vegan Society have cited the book as the first vegan cookbook.