 Thornley was born in St Asaph and brought up in Wrexham.She began her sporting career through the "Sporting Giants" programme, becoming the first of the scheme's graduates to win a gold medal when she was successful at the 2009 World Under-23 Championships.In the 2016 Summer Olympics, she was partnered with Katherine Grainger in the women's double sculls, in which they took the silver medal.