The 33-year-old takes charge at The Hawthorns on a three-year contract, succeeding Tony Mowbray, who was relieved of his duties in April.
Mason joins the Championship club following a seven-year coaching spell at Tottenham Hotspur, where he rose through the ranks after being forced into early retirement due to injury.
The former England international now faces the challenge of guiding West Brom back to the Premier League, with the Baggies narrowly missing out on the play-offs last season.
A product of Tottenham's academy, Mason joined Spurs at the age of eight and progressed through the youth system to make his first-team debut in 2008.
After several loan spells including stints at Yeovil Town, Doncaster Rovers, and Swindon Town, he finally broke into the Spurs first team under Mauricio Pochettino during the 2014/15 season.
He made 70 appearances for Tottenham, including 53 in the Premier League, and scored four goals.
Mason's tenacity and work rate in midfield earned him his only senior England cap in March 2015, coming on as a substitute in a friendly against Italy.
In August 2016, he joined Hull City for a then-club-record fee of around £13 million. However, his playing career was tragically cut short in January 2017 when he suffered a fractured skull in a Premier League match against Chelsea.
After a long recovery process and medical advice, Mason retired from professional football in February 2018 at the age of 26.
Shortly after retiring, Mason transitioned into coaching and rejoined Tottenham in April 2018, taking a role in the club's academy. His intelligence, leadership, and connection with Spurs quickly earned him promotions within the setup.
In April 2021, he became the youngest manager in Premier League history when, at just 29, he was appointed interim head coach following the dismissal of Jose Mourinho prior to the EFL Cup final.
He guided Tottenham to four wins in six games, including a 2-0 victory over Wolves, and helped secure European qualification.
Mason remained a key figure on the first-team coaching staff under subsequent managers, including Nuno Espirito Santo, Antonio Conte, and most recently Ange Postecoglou. Following Conte's departure in March 2023, Mason was again elevated to the role of acting head coach, overseeing the final six matches of the 2022/23 season.
Though he returned to a senior coaching role under Postecoglou, his contract with Tottenham was set to expire at the end of June.
Now embarking on his first permanent managerial role, Mason expressed his enthusiasm about the opportunity at West Brom:
"This is a huge club with a fantastic infrastructure and an incredible fanbase, and I am excited about what we can achieve together," he said.
"Having spoken at length to the board and those at the club, I am convinced that Albion is the perfect place for me to be and I can't wait to get started.
"I will bring with me a huge amount of enthusiasm, dedication and ambition, and look forward to a positive future together at such a fantastic club."
Mason will be joined by Nigel Gibbs as assistant coach, who most recently worked at Reading, while Sam Pooley joins from Rangers as first-team coach and head of performance.
As West Brom aim to mount a serious promotion challenge in the 2025/26 season, all eyes will be on Mason to see if he can translate his coaching experience from north London into success in the West Midlands.