Giuntoli was born in Milwaukee, Wisconsin, to Mary and David Giuntoli. He is of Italian (father) and Polish and Austrian (mother) descent. He was raised in Huntleigh, a suburb of St. Louis, Missouri. He admits to not being very athletic as a child. In an Access Hollywood interview Giuntoli stated "I was like three feet shorter than I am now until I was a sophomore in high school. But I had the same sized head and ass as I do now. I was an awkward little fellow. In high school I started gravitating a little toward acting." After graduating from St. Louis University High School in 1998, he attended Indiana University at Bloomington, earning a bachelor's degree in International Business and Finance in 2002.
Giuntoli returned to St. Louis after college. However his heart was in being an entertainer instead of a career in finance, something his family realized early on as he always enjoyed making people laugh from a young age. Reconnecting with his high school theater teacher, he began taking acting lessons locally.
His first career break came in 2003 when he was discovered by talent scouts for MTV and cast in the networks reality series Road Rules: South Pacific. The three-month adventure in the South Pacific, and a subsequent appearance on the seventh season of Real World/Road Rules Challenge provided the resources to pay off his college debt and further cemented his decision to pursue acting as a full-time career.
In 2007, he moved to Los Angeles to further pursue an acting career. There he studied under director and acting teacher Chris Fields before joining the Echo Theater Company. A long series of guest appearances on various TV series and TV movies followed, including Nip/Tuck, Veronica Mars, Grey's Anatomy, Ghost Whisperer, Privileged, Without a Trace, and Cold Case among others. Giuntoli was also in consideration for the title role of the relaunched Superman movie Man of Steel, but was eventually beaten out by English actor Henry Cavill.
His biggest role in TV to date is the series Grimm, which premièred in October, 2011. He played the lead role of Nick Burkhardt, a Grimm detective who hunts monsters. In addition to his work on Grimm, Giuntoli was seen in 2012 in the motion picture Caroline and Jackie in a supporting role. He played the role of Scott Wickland, DS agent, in the 13 Hours directed by Michael Bay. In 2016, he played the lead role of David in the comedy film Buddymoon.
In December 2014, his Grimm co-star, Bitsie Tulloch, confirmed they are in a relationship. In July 2016, he and Tulloch revealed they became engaged in April 2016. They married in June 2017.
Giuntoli currently lives and works in Portland, Oregon, where Grimm was filmed. He often rode his bicycle to the set. During a 2012 break in Grimm production, Giuntoli visited an elephant orphanage in Kenya, adopting one of the animals.