General Discussion

Sums it up nicely... he's as mad as a hatter.

Its earliest known appearance in print dates back to 1829 in Blackwood's Edinburgh Magazine, well before Lewis Carroll's character in Alice's Adventures in Wonderland was introduced in 1865.

Theories Behind the Phrase
Several theories explain the origin of the phrase:

Mercury Poisoning: In the 18th and 19th centuries, hatmakers often used mercury in the production of felt hats. Prolonged exposure to mercury fumes led to symptoms like tremors and irrational behavior, which contributed to the association of madness with hatters.
Etymological Roots: Some linguists suggest that "mad as a hatter" may derive from an older phrase "mad as an atter," where "atter" is an Old English term for "adder" (a venomous snake). This interpretation implies a meaning of being "venomous" rather than "crazy."