A mud clerk was a helper or all-around worker aboard a steamboat

Steamboat

A steamboat or steamship, sometimes called a steamer, is a ship in which the primary method of propulsion is steam power, typically driving propellers or paddlewheels...

 during the period before and after the American Civil War

American Civil War

The American Civil War was a civil war fought in the United States of America. In response to the election of Abraham Lincoln as President of the United States, 11 southern slave states declared their secession from the United States and formed the Confederate States of America ; the other 25...

, particularly aboard steamboats on the Mississippi River

Mississippi River

The Mississippi River is the largest river system in North America. Flowing entirely in the United States, this river rises in western Minnesota and meanders slowly southwards for to the Mississippi River Delta at the Gulf of Mexico. With its many tributaries, the Mississippi's watershed drains...

.

According to Mark Twain

Mark Twain

Samuel Langhorne Clemens , better known by his pen name Mark Twain, was an American author and humorist...

 in his autobiography, "Mud clerks received no salary, but they were in the line of promotion. They could become, presently, third clerk and second clerk, then chief clerk -- that is to say, purser

Purser

The purser joined the warrant officer ranks of the Royal Navy in the early fourteenth century and existed as a Naval rank until 1852. The development of the warrant officer system began in 1040 when five English ports began furnishing warships to King Edward the Confessor in exchange for certain...

".

Mud clerks were always male, and typically in their early teens or younger. Duties included such things as running errands for the officers of the steamboat, carrying messages around the ship, and fetching food or beverages. As the name itself implies, mud clerks would often be given the dirtiest jobs aboard ship.