The most memorable mission of Project Mercury was Freedom 7.

An excerpt from the article 29 facts about NASA
The liftoff took place on May 5th, 1961. It allowed Alan Shepard Jr. to go down in history as the first American in space. He was the second man in the cosmos, however, after Yuri Gagarin, who orbited Earth not a month earlier, on April 12th, 1961.