|Status||Living, presumably on the run|
|Relationships|| Harcourt Grimesby (twin brother)|
Roderick Grimesby (father)
Mycroft Grimesby and Harcourt Grimesby were the twin sons of Roderick Grimesby. When they were born the family did not have the means to support two children. Roderick and his wife kept Harcourt and gave up Mycroft.
As an adult, Mycroft became a ventriloquist or "belly speaker" using his brother's name Harcourt and has a puppet named Mycroft.
He murdered his father and was briefly incarcerated at Station House Four. He was released when he told Detective William Murdoch that his twin brother, Mycroft, had murdered their father. By the time Murdoch uncovered the truth, Mycroft is untraceable.
The Grimesby men have the genetic trait known as heterochromia in which the eyes are different in colour. Where Roderick and Harcourt had brown right eyes and blue left eyes, Mycroft had the reverse. His puppet was painted similarly.
- Harcourt Grimesby (his twin brother, killed by unspecified causes, assumed his identity afterwards)
- Roderick Grimesby (his father, forced to drink varnish)
- The name Mycroft is a reference to Sherlock Holmes' fictional brother, Mycroft Holmes. The creator of Holmes, Arthur Conan Doyle, appears in this episode.