In 1897, having been appointed assistant district commissioner in the Punjab, Douglas was required to investigate the allegations of
Henry Martyn Clark against
Mirza Ghulam Ahmad. Clark, a Christian missionary, had been approached by a youth named Abdul Hamid, who claimed that Ahmad had sent him to kill Clark. Douglas found Hamid's claims to be implausible, and that there was evidence that Hamid had been coached. He had also repeatedly changed and even retracted his story. Douglas dismissed the charges. Even after more than forty years he vividly remembered the case and its details. He wrote to J. D. Shams, an
Ahmadiyya missionary in London on 29 July 1939, "... the evidence was false and thus I acquitted Mirza Ghulam Ahmad". The aftermath was presented by Ahmad's supporters as a triumph, and as evidence of Ahmad's divine mission. Subsequently, Ahmad compared Douglas favourably with
Pontius Pilate, declaring him to be a man of much superior character, stating, In my opinion, Captain Douglas outshines Pilate in imparting judgment fearlessly and in showing determination and steadfastness ... Pilate showed cowardice due to fear of the High Priest and the Elders, and acted in a cowardly manner. But Captain Douglas showed no sign of weakness ... Those who are blessed with honour from above do not hanker after worldly honours. This commendable courtesy by the 'Pilate' of our time shall be remembered by all my followers when I am gone. He shall be remembered till the end of the world with love and respect."In his book
Kitab ul Baryyah (An Account of Exoneration) Mirza Ghulam Ahmad has narrated the details of the case. ==Later career and retirement==