Who is Luffy's first mate? I just started reading the One Piece manga and am currently on the 5th and I was wondering who Luffy's first mate is.

Zoro joined first, but Usopp was given permission to take over as captain when Luffy couldn't, which is something only the first mate can do.

Who is Luffy's first mate?
Well Mugiwara no ichimi has no first mate but according to One Piece Yellow: Grand Elements, Roronoa Zoro is referred to as being "like the vice captain". Bartolomeo has also referred to Zoro as the first mate of the Straw Hat Pirates, although, by his own admission, that was just an assumption based on his personal views of the crew.

link here https://onepiece.fandom.com/wiki/First_Mate