Originally, he intended to make a music career. Instead, his widowed mother persuaded him to take up banking, which he reputedly hated. For a year or so, he travelled throughout Europe, earning a living playing piano in cinemas and elsewhere. He won an appointment teaching English at Heidelberg University.
At the outbreak of the First World War, he was sent as a clandestine war correspondent to France. Journalists were not allowed in the war zone at this time, but Clarke managed to evade the authorities longer than any other reporter to roam the front lines. Years later, war correspondents had become greatly respectable and he travelled to almost every theatre of war reporting for the Daily Mail.
In Dublin in 1920 the propaganda apparatus pumped out entirely false and deliberately misleading stories. 'Propaganda by news' was how they described it. The key quality that it must have, according to Basil Clarke who was in charge of the operation, was 'verisimilitude' - having the air of truth. According to his own account the routine 'issue of news gives us a hold over the press'. At the twice-daily press briefing at Dublin Castle, journalists 'take our version of the facts' and they believe all I tell them', wrote Clarke. The service 'must look true and it must look complete and candid or its "credit" is gone'. The policy, therefore was to disseminate lies and half truths which gave the appearance of truth. As Major Street, another of the propagandists noted: 'in order that it may be rendered capable of being swallowed', propaganda 'must be dissolved in some fluid which the patient will readily assimilate'."
He left government service in the early 1920s and set up one of the first PR agencies, Editorial Services, in 1924. By the end of the 1920s he was running a significant operation with 60 staffers. The same year he was associated with "the setting up of ‘National Propaganda’, later to become ‘The Economic League’.
During the reign of King George V, Clarke was asked to write several speeches for the monarch. George V apparently once remarked "Clarke, I like the speeches you write for me, you don't make me sound too bloody pompous."
He is credited with making pasteurised milk acceptable in England and campaigned for legislation to have imported skimmed milk marked "unfit for babies." On behalf of the Heinz organisation, he successfully fought for legislation to stop the use of harmful colouring matter and adulterants in preserved foods. Henry J. Heinz, the founder of the business, was personally brought over from America to give evidence at a select committee of the House of Commons on the subject.
The Danish government, for his services in "promoting Anglo-Danish friendship and trade," awarded him the Order of the Dannebrog. He was also made a Knight of the Realm during the brief premiership of Andrew Bonar Law.