The Teenage Mutant Ninja turtles were created in 1984 by Kevin Eastman and Peter Laird. But it turns out that the roots of the story about the four mutated-turtles, who assume the name of Renaissance-era artists and don masks to fight crime, could go much further back than that.
After getting a look at the face of the newest ninja turtles, a question came up: Why the masks?
You know what has always bothered me about the Turtles? (I mean, among the great many things that bug me about everything)
Why do they wear masks? Are they afraid someone might recognize them?
Officer: Do you think you could describe them?
Citizen: Uh, they kinda looked like turtles, you know? Round shell. Bald. Kinda green. Spots. No real nose.
Officer: Could you pick them out of a lineup?
Citizen: I doubt it. They were wearing masks.
They were created as a parody of Frank Miller's ninja characters from the early eighties, so they wore the trappings of "comic book ninjas." Now it's just part of the look. Originally, the masks were all black, but they added color for the cartoon. Even the later comics had them all be red masks.
Eastman also backs up the tale of Miller's (along with some additional artists') influence, in this account in Heavy Metal, where he is the publisher:
Okay—the real story—late one night I did this drawing of a Turtle, mask and weapons in place, posed for battle (below)—all to make Pete laugh . . . Anyway, so Pete and I kicked around this idea of a bunch of "Teenage Mutant Ninja Turtles" (stolen from "Cerebus," "Ronin," "New Mutants," and "Daredevil") wrote a story to tell how they came to be, (all on our very own!) and decided to sell it through the direct market Phil Seuling created, borrow a bunch of money from my uncle, and see what happened.