|Portrayed by||Rhys Ifans|
|First seen||Step Nine|
|Last seen||The Grand Experiment|
|Status||Alive, (Fake dead).|
|Nickname(s)||Fatty (by Sherlock)|
|Residence||221B Baker Street, London, England|
|Significant other(s)||Nigella Mason (ex-fiancée)|
Mycroft Holmes is Sherlock Holmes' older brother, and M.Holmes 'older son. He works as a restauranteur and an operative in MI6. He has been estranged from Sherlock for years, due to Sherlock's affair with Mycroft's fiancé. Mycroft suffered from Leukemia, and now he's looking to improve his relationship with Sherlock. Mycroft also cares deeply about Joan.
He is the current resident of 221B Baker Street London.
|Season two appearances|
|Step Nine||Solve for X||We Are Everyone||Poison Pen|
|Ancient History||An Unnatural Arrangement||The Marchioness||Blood Is Thicker|
|On the Line||Tremors||Internal Audit||The Diabolical Kind|
|All in the Family||Dead Clade Walking||Corpse de Ballet||The One Percent Solution|
|Ears to You||The Hound Of Cancer Cells||The Many Mouths of Aaron Colville||No Lack of Void|
|The Man With the Twisted Lip||Paint It Black||Art in the Blood||The Grand Experiment|
- He owns a Michelin-star restaurant where he invited Joan Watson for dinner in order to understand how to "be Sherlock's friend" in 'Step Nine'
- He opened a new restaurant in New York named Diogenes, named after the Club Mycroft in Conan's fictions frequented in London.