At the outbreak of World War I, Lionel's younger brothersEvelyn and Anthony both joined the British Army. However, as the eldest son he was needed as the heir to take over the family's N M Rothschild & Sons banking house. Much to his frustration, Lionel had no choice but to remain at home. When Britain entered the War in August 1914, Lionel had already been promoted from captain to major in the Royal Buckinghamshire Yeomanry. As the Government recognized that many Jews yet had not been enlisted in military service, a Central Jewish Recruiting Committee was established in December 1915. Its office was located at the Rothschild's New Court, which was colloquially known as "Rothschild’s Recruiting Office", while Lionel was appointed vice-chairman. Chairman was Edmund Sebag-Montefiore. In 1917 he co-founded and headed the anti-Zionist League of British Jews. Lionel was made OBE in the Military Division in 1917. He was retired from the regiment in 1921. Both of his brothers were wounded in battle, and brother Evelyn died of combat injuries suffered at the 1917 Battle of Mughar Ridge.
Engagement in plants
His father, Leopold, died in early 1917 and Lionel and brother Anthony became the managing partners of N M Rothschild & Sons bank. However, Lionel de Rothschild had developed an interest in horticulture at a very young age and is said to have planted his first garden at the age of five. In 1919, he purchased the Mitford estate at Exbury in Hampshire, where he devoted a great deal of time and money to transform it into one of the finest gardens in all of England, with more than one million plants. In the 1920s, he built Exbury House around an existing structure in a neo-Georgian style. He constructed a private railway to transport rocks to build the largest rock garden in the country. Lionel de Rothschild also co-sponsored plant-hunting expeditions to places as isolated as the Himalayas to collect seed for plant growth and experimentation. In all, he developed 1,204 new hybrids of rhododendron and azalea that were recognized and sold around the world. Although he continued to work at the family bank, he is quoted as describing himself as "a banker by hobby — a gardener by profession".
Death
Lionel Nathan de Rothschild died in London, aged sixty, in 1942 and was buried in the Willesden Jewish Cemetery. His son Edmund took over management of the Exbury Gardens and would eventually create a charitable trust to manage the property. In 2001, the American Rhododendron Society recognised Lionel Nathan de Rothschild's significant contribution, posthumously bestowing on him a Pioneer Achievement Award.