Although he had written fiction since the 1890s, Fletcher found his true vocation in crime writing, beginning with The Root of All Evil (1911). His breakthrough came with The Middle Temple Murder (1918), a classic mystery that became an international bestseller and was famously admired by U.S. President Woodrow Wilson. Its success established him as a master of the police procedural, and he followed it with popular titles such as The Talleyrand Maxim (1919), The Orange-Yellow Diamond (1920), The Borough Treasurer (1921), The Charing Cross Mystery (1923), and Murder in Silesia (1932). His stories are noted for realistic settings, intricate plotting, and engaging characters, often journalists and detectives unraveling complex cases.
Beyond crime fiction, Fletcher wrote extensively on Yorkshire history and rural life, and his historical scholarship earned him a Fellowship of the Royal Historical Society.