Was there a character named Mordecai in the Sherlock Holmes book or movies?

There is no character named Mordecai in the Sherlock Holmes stories written by Sir Arthur Conan Doyle. Mordecai is a common name, but it doesn't appear in any of the original Sherlock Holmes novels or short stories.

Perhaps you are thinking of a different character, or a character from a different adaptation of the Sherlock Holmes stories?