I recently read A Short History of Charleston and there is a constant reference to a class of people dubbed "Mechanics."
The portion of the book that refers to Mechanics covers pre-civil war Charleston but doesn't specifically define the class of people. The lines are somewhat blurred between artisan slaves who paid their own cost to their owners to have a semblance of freedom and immigrants with no social standing.
I thought I'd post here before I go back and try re-reading the book to figure this out as Google hasn't provided much other than the game mechanics of Civilization V's slavery.
I can't comment specifically on Charleston but prior to mass industrialisation the term 'mechanic' was synonymous with 'artisan' or 'craftsman'. Essentially a mechanic was a member of the petite-bourgeoisie who engaged in manual labour. As such it didn't include slaves, those without property or, as you note, recent immigrants. Indeed, with regards the latter, a number of mechanic societies were at the forefront of the Nativist movement in the 19th C US (eg, Order of United American Mechanics).
Interesting, the term itself is pretty old. I've seen references in Christopher Hill's works on the English Civil War to 'mechanick preachers' in the early 17th C. These were artisans who, despite being relatively uneducated in theology, served as lay preachers on a Sunday.