Mycroft Holmes is the elder brother of the famous Sherlock Holmes and later M. Like his brother, Mycroft shares a deductive trait but is known for being sedentary. He is also the co-founder of the Diogenes Club, where he usually stays.
In the years that followed his brother's supposed death, Mycroft was extremely warily of James Moriarty's rise to head of MI5 in which his fears became not unfounded when the megaloamical Moriarty attempted to bomb London in 1898 in his war against The Doctor. Mycroft ultimately succeeded Moriarty after the professor was declared "missing", and became the new confidant of The League. It is hinted that he is aware of Sherlock Holmes' factual survival.
By the events of the Martian invasion, Mycroft oversaw Britain's military strategy against the invaders. During a meeting with the League, Mycroft referred to the invasion as a war, as calling it an invasion to the public would cause a massive panic throughout the country. After the early developments of the invaders's introduction of tripods and the Invisible Man's betrayal, Mycroft keenly analyzed in using Dr. Alphonse Moreau's biological weapon H-142 against the alien invaders as the ultimate resort. He dispatched Mina Murray and Allan Quatermain in finding Moreau. The acquiring of H-142 and its use came into fruition, but also consequently served in breaking up the League due to its unethical use that also killed any human bystanders that were unfortunately exposed to the virus.
Mycroft continued his service and later became the confidant of the early 20th century League. In 1910, Mycroft was debriefed on Thomas Carnacki's disturbing visions that coincides with the upcoming coronation of King George V. Mycroft dispatched the League in investigating these matters, and especially concerning that the visions may also connects to the serial killer, Jack MacHeath, for the Whitechapel murders. After MacHeath was caught and about to be hanged without trial, Mycroft worried that a trial might bring to light the involvement of the 14th Earl of Gurney in the original Whitechapel murders. But at the last moment, a message arrived from the Earl of Gurney confessing to all the Whitechapel crimes and thereby exempting MacHeath much to Mycroft's outrage.
Mycroft Holmes was created by Arthur Conan Doyle and first appeared in The Adventure of the Greek Interpreter.