No one person discovered the planet Mercury. It has been observed for thousands of years, and although people might not have known what it was, they knew of its existence as early as 3000 B.C.
Mercury can be seen with the naked eye. Unfortunately, because it is never far from the sun, this makes observing it a difficult task because it is always so dark. Scientists believe that it was not recognized as a planet until Galileo explored the universe with his telescope sometime during the 1540s. Although Copernicus was the first to realize the sun was the center of the galaxy, not the earth, Galileo was able to confirm his theories after he invented the telescope.