Mark was born in Maynooth, Republic of Ireland in May 1999 and played for a number of Irish clubs at youth level.

The goalkeeper made the move to England with Bournemouth as a 17-year-old and made his first senior appearance on loan at Weymouth, remarkably scoring on debut.

He first appeared in competitive action for Bournemouth in May 2019, keeping a clean sheet in a 1-0 Premier League victory over Tottenham Hotspur. He won the Player of the Match award.

Mark made eight appearances on loan at Swindon Town in 2021 and also had a temporary spell at Stoke City two years later.

He was capped from Under-15 to U21 level with the Republic of Ireland and has also won four senior caps for his nation.

He was Bournemouth's first-choice goalkeeper as the Cherries won promotion to the Premier League in 2021/22 and in total has made 82 appearances for the club.

Boro signed him on loan in January 2025.