Tomlinson was born in Doncaster, South Yorkshire, to Johannah Poulston and Troy Austin, who split up when he was a toddler. He is estranged from his father. He later took on his then stepfather Mark Tomlinson's name. He has seven half siblings. His mother died, after a short battle with leukaemia, in December 2016. His half sister, Félicité, died in 2019, as a result of cardiac arrest. Tomlinson was a sixth form student at Hall Cross School and is a former pupil of the Hayfield School. He failed his first year of A levels at the Hayfield School and ended up going back to Hall Cross and starting A levels again. He had a number of jobs, including at a Vue cinema and at Doncaster Rovers football stadium as a waiter in the hospitality suites. Tomlinson, along with two of his siblings, had extra roles in Fat Friends. After Fat Friends, he attended an acting school in Barnsley. He had small parts in an ITV drama film If I Had You and BBC's Waterloo Road. At Hall Cross, Tomlinson appeared in several musical productions. He admitted to not applying himself academically in school, but he thoroughly enjoyed being with his friends and being involved with musical productions. Taking the lead role of Danny Zuko in the Hall Cross musical production of Grease motivated him to audition for The X Factor.