Root is the elder son of Helen and Matt Root and grew up in Dore, near Sheffield. He has a younger brother Billy, who plays cricket for Nottinghamshire. Root became engaged to his girlfriend Carrie Cotterell in March 2016; their son, Alfred William Root, was born on 7 January 2017. He supports football team Sheffield United.

He attended King Ecgbert School, Sheffield and at 15, on a cricket sports scholarship, Worksop College as a weekly boarder.

Root followed in his father's footsteps by joining Sheffield Collegiate CC. Former Yorkshire batsman and England captain Michael Vaughan also learnt his trade at Collegiate and was a source of inspiration for Root, who became a protégé of his.

Root made his Yorkshire Second Team debut on 18 July 2007, against Derbyshire at Abbeydale Park. He scored 57, putting on 133 for the first wicket with Adam Lyth. He continued to represent the Academy side and was named Player of the Tournament as Yorkshire's Academy won the ProARCH trophy in Abu Dhabi.

After success at second team level, Root was given an opportunity in the first team in Yorkshire's final Pro40 match of the season at Headingley, against Essex. Root scored 63 and was top-scorer in Yorkshire's 187–7. Although his half-century could not inspire Yorkshire to victory, Root described his debut as a "dream come true".

After another Man of the Series performance, this time on England Under-19 duty in Bangladesh, Root signed a three-year professional contract with Yorkshire.