The Warrior Mullah provides a historical account of Mohammed Abdille Hassan, who conducted a twenty-five-year campaign to keep Somaliland free from European and Ethiopian influence. Hassan, referred to by the English as the "Mad Mullah," is described as a man of immense charisma and a master of desert guerrilla warfare, driven by religious fervor after joining the puritanical Salihiya brotherhood during a pilgrimage to Mecca.

The narrative details the rise of the Dervish movement and the subsequent military challenges faced by the British administration. It explores the British perspective as "reluctant colonizers" whose primary objective was to prevent other European powers or Ethiopia from controlling the region following the Egyptian withdrawal, primarily to protect the sea route to India.

The book tracks several distinct periods of conflict, including the early "Expedition Period" (1900–1905), the withdrawal to the Somali coast (1910–1913), and the significant reversal of British policy leading to the final "21-Day Campaign" in 1920. This final campaign was notable for the introduction of the Royal Air Force as a decisive factor in military operations.

Ultimately, the work concludes with the death of Hassan in 1920 and an assessment of his legacy as a national hero to the Somali people. It reflects on the "tragic story of unnecessary deaths and waste" and the eventual unification of the Somali Republic in 1960.