Brennig is from a working-class background in South Wales; despite being the grandson, son and brother of steel workers he opted to be the first in his family to go to university and enter the TV/Film industry. Brennig was shortlisted for the BBC Wales Writer in Residence before being accepted onto the 2021-22 BBC Writersroom Drama Group. In the winter of 2023, he had his first broadcast credit as a director, with his short film 'Back to the Romance' airing on BBC2.