Joseph Raymond McCarthy was born on a farm in the Town of Grand Chute, near Appleton, Wisconsin, on November 15, 1908. He attended the Underhill School, a one-room schoolhouse, where he completed eighth grade. Bored with farm work, McCarthy started his own chicken business as a teenager, but disease wiped out his flock. Broke at age 20, he worked as a clerk in an Appleton grocery store, quickly becoming manager.

In 1929, McCarthy was transferred to Manawa to manage a new grocery store. While there, he entered Little Wolf High School, completing the four-year curriculum in nine months. McCarthy’s excellent grades enabled him to attend Marquette University in Milwaukee, which he entered in the fall of 1930. In school, he coached boxing, and was elected president of his law school class, all while working a series of part-time jobs. Immediately after gaining his law degree in 1935, McCarthy opened a practice in Waupaca. He later joined a law firm in Shawano, becoming a partner in 1937.