I had the same question before I read this article. Wally Yonamine was known as "Nisei Jackie Robinson". He was the first American to play in the Japanese baseball league after WWII and was the first American to be inducted into the Japanese Baseball Hall of Fame.
Before playing baseball, though, he was the first Japanese-American to play professional football...he played for the 49ers just after WWII. His career was short-lived lasting only one year. It was then Yonamine turned his attention to baseball.
You may not find this as interesting as I did but I think it's a good read from a historical standpoint.