The New York Yankees first added pinstripes to their uniforms for the 1912 season. At the time, they were still known as the New York Highlanders, though the name “Yankees” was becoming more and more popular, and became the official name the next year.
For the next two years, the Yankees shared the Polo Grounds with the New York Giants, and the pinstripes were taken off of the home uniforms. Then, in 1915, they returned, and have not been removed since.
The myth that Colonel Jacob Ruppert had pinstripes added in the 1920s to make Babe Ruth look thinner is untrue.