Nia Roberts was born on 5 July 1972 and raised in Brecon, Powys, the youngest of three sisters in a Welsh-speaking family. Her family were active members of the Brecon Little Theatre amateur dramatics group, and she made her first stage appearance at the age of seven. She was educated at Brecon's Welsh-medium primary school, Ysgol-y-Bannau, before going on to study drama at Birmingham University, where she graduated with a BA Honours degree, with distinction.
Her first television role came in 1984, playing the lead in a thirty-minute Jackanory production of The Farm for BBC1. Her breakthrough came in 1999 when she starred opposite Ioan Gruffudd in Solomon a Gaenor, a Welsh and Yiddish-language drama that was nominated for Best Foreign Language Film at the 2000 Academy Awards. Her performance in Lois won her the Best Actress award at the 2000 BAFTA Cymru ceremony; she was nominated again the following year for Solomon a Gaenor, and again in 2024 for Pren ar y Bryn.
Roberts has built a substantial career across Welsh and English-language film and television, including the BBC Wales hospital drama Crash!, a two-part guest role in Doctor Who in 2010, and film roles in Snow Cake and Patagonia, both directed by her husband, Marc Evans. Her more recent credits include Hinterland, Keeping Faith, Hidden, an episode of The Crown, and the BAFTA Cymru-nominated drama Pren ar y Bryn.
She has continued to work extensively in Welsh-language drama throughout her career, including long stretches on Pobol y Cwm and the S4C gangster series Y Pris, and remains closely associated with the Brecon community where she grew up.