He was named for a maternal uncle, Joseph William Green Jackson, who died in the American Civil War.

He then enrolled at Vanderbilt University, where he won honors in English and history, actively participated in debates, and became a member of Beta Theta Pi fraternity.

[1] Byrns displayed a strong early interest in politics and was elected to the Tennessee House of Representatives in 1894 and reelected in 1896 and 1898.

In 1902, he ran for district attorney of Davidson County, Tennessee, but was defeated—his only unsuccessful political race in 18 efforts.

In 1908, Byrns received the Democratic nomination for U.S. Representative and was elected in November of that year to a term beginning March 4, 1909.