Captain America was the first patriotically-themed superhero, making his debut in his own Captain America Comics in March of 1941. The character was predictably popular during the War, but afterward started to fade into the background (as nearly all wartime superheroes did).
The character was revived in Avengers #4, March 1964, after about a decade of not being published. The newly arrived Captain joined the super-team and quickly became one of Marvel's most popular characters, later graduating to his own book and gaining a colorful collection of friends and enemies.
The Mego Captain America action figure came with a round shield, as the comic book version did; the toy shield was made of black plastic, with a sticker placed over the front.