Captain America was not an original member of the Avengers.
Although today it would be tough to imagine the Avengers without Captain America, he wasn’t always part of the team. There are many ways in which the story is portrayed, however, the original lineup did not include him. The fourth issue of The Avengers comics from 1964 saw the already formed team actually found him buried in ice. He only joined the original crew of Ant-Man, Thor, Iron Man, Wasp, and the Hulk after the team kicked the Hulk out for his unpredictable behavior and uncontrollable powers. He was awarded the status of founding member with the addition of Hawkeye, Quicksilver and the Scarlet Witch.