Post-1900 is not a period I generally like to read about, but the story of Juan Pujol Garcia is one I found fascinating. If the Wikipedia article about him can be trusted:

After serving in, becoming disillusioned by, and deserting from both sides in the Spanish Civil War, when World War II started he decided he needed to do something to help the Allies "for the good of humanity." He volunteered to become a British spy on the Germans three times, also contacting the Americans, and was turned down each time ... so he constructed a false identity, volunteered his services as a spy to the Germans, and started feeding them false information.

His German handlers had instructed him to move to London and develop a network of agents in Britain. Instead, he moved to Lisbon, Portugal, and made his reports based on British travel guides, research in public libraries, newsreel footage, and other public information. He invented fictitious agents under him whom he blamed for false information and mistakes, and submitted fictitious reports of his supposed activities in Britain, including filing for reimbursement for expenses incurred in extensive travel throughout Britain. (Expense reports which were complicated by the fact that he didn't understand the British currency system of the time, pre-decimalization ...) Somehow, despite being a combination of public information and his own fabrications, his reports were convincing to both his German handlers and British intelligence, who launched a large-scale spy hunt after intercepting some of his reports.

Eventually he got in touch with the British intelligence service, who finally accepted him as an agent, brought him to Britain, and arranged to work with him to coordinate the passage of false and useless intelligence to the Germans---including information that would have been useful artificially delayed long enough to be useless. With the help of his British handler, his reports to the Germans became so voluminous that the Germans apparently became overwhelmed and stopped trying to recruit more spies in Britain.

The Germans became increasingly frustrated with the delays, and so sent him radio and encryption equipment; he promptly shared the encryption equipment with the British codebreakers at Bletchley Park, enabling them to carry out even better known-plaintext attacks on the Enigma encryption than the German bureaucratic system had previously allowed (e.g. from a set header for daily orders and reports).

In the lead-up to the D-Day invasion, he sent misinformation to the Germans as part of the British "Operation Fortitude," helping to mislead the Germans about the invasion site, and then after D-Day about the size of the invading army to convince them the original landing was a diversion. On D-Day itself the incompetence of the German radio operators he was reporting to, who didn't reply to his message for five hours despite having previously arranged to be waiting for an urgent message, allowed him to again send accurate information too late to be useful.

Over the course of the war, he was paid "$340,000" (I presume that's US dollars of the period) for himself and his fictitious agents. After the war, he was awarded an MBE, and because the Germans never realized they had been fooled, he was also awarded the Iron Cross.

After the war, he traveled to Angola, faked his death from malaria, and moved to Venezuela. There he lived in anonymity until a British author managed to track him down, in part by calling every "J. Garcia" in the Barcelona phone book until he found a Juan Pujol Garcia's nephew.

He died in 1988.