Justin was born in Oakville, ON and has an identical twin to Tyler. Justin is the younger of the Donati twins by 7 minutes.
On January 8, 2005, Justin was acquired from the Oshawa Generals to the Toronto St. Michael’s Majors, along with his brother Tyler. In February 2007, he was acquired by the Sudbury Wolves (Tyler was traded to the Belleville Bulls). Neither brother was drafted to the NHL in 2006, but both attended Wings rookie camp. Both brothers played well, producing points regularly throughout the tournament.
In 2007, Justin was invited to participate in the Calgary Flames rookie camp as a non-draft invitee. He signed with the (UPE) Panthers of the Atlantic University Sport conference in December 2007. A shoulder injury kept him out until early February, but he posted 10 points in his four games with the club.
2011 saw Justin winning the ECHL scoring title with 27 goals, and a league best 67 assists for 94 points in 63 games. He was with the Elmira Jackals at the time, his nearest rival had 83 points.
On May 31, 2013, Justin left Norway and joined his brother in signing a one-year contract with the Austrian club, HC TWK Innsbruck of the EBEL. A free agent after a single season with Innsbruck, Donati signed a one-year contract in Sweden with Vita Hästen of the HockeyAllsvenskan on July 10, 2014.
In December 2014, Justin signed a contract with the Brampton Beast, reuniting him with his brother for the third time in his career.
Justin and his brother Tyler are actively involved in the "points for Cancer" fund, with money for each point either Donati scores being matched by their respective hockey clubs going to research to finding a cure for ovarian cancer, which their mother Corinne was diagnosed with in 2003. She died in July 2006. The "points for Cancer" fundraising continues in her name.