Darnell impressed at youth level at Queens Park Rangers, signing his first professional contract with the Club in 2014. He made his senior debut in the Premier League in 2015.  

Darnell impressed at youth level at Queens Park Rangers, signing his first professional contract with the Club in 2014. He made his senior debut in the Premier League in 2015, aged 19. 

He gained experience on loan during the 2015/16 season at Cambridge United and Northampton Town and began the 2016/17 campaign on loan at Swindon Town, before being recalled to the QPR set-up by Ian Holloway. 

Darnell established himself as a First Team regular upon his return to the Club, appearing as a right back and centre back for the side. In total, he made 72 appearances for the Club. 

Darnell joined West Bromwich Albion in Summer 2019 on a four-year deal.