Reynor was born in Colorado, United States. His mother, Tara (Reynor) O’Grady, is a human rights defender from Ireland. Jack moved to County Wicklow, Ireland, when he was two years old. He was raised there by his mother and maternal grandparents, Pat and Damien Reynor. His interest in acting began on the set of Country, directed by Kevin Liddy, in 1999. Between 2004 and 2010, he attended Belvedere College SJ in Dublin, where he performed on stage in numerous productions. In early 2010, he was cast in Kirsten Sheridan's "Dollhouse" which debuted at Berlinale 2012. In late 2011, he played the title role in Lenny Abrahamson's highly anticipated What Richard Did.