While still a player, Tandy first started out coaching with his boy-hood club
Tonmawr RFC, leading their seconds (Tonmawr Dragons) to win the Percy Howell’s Districts Cup. He also coached within the Ospreys academy, leading the U16s to a WRU Age Grade Championship title in 2010. During the 2010/11 season, he continued to combine his playing role with the Ospreys with the head coaching role at
Bridgend Ravens, where he led the Ravens to the WRU Division 1 West title, with an incredible 96 points (just one defeat and one draw shy of an unbeaten season). He later guided them into the
Principality Premiership with a victory over
Glamorgan Wanderers (38–19) at the
Cardiff Arms Park in the play-offs. By the start of the 2011/12 season, he had played his last game for the Ospreys and had officially retired from professional rugby. With this, he continued to coach the Ravens, whilst also acting as a technical coach for the Ospreys and acting head coach for the Ospreys during their
Anglo-Welsh Cup campaign. However, in February 2012 he left the Ravens after he was installed as the Ospreys head coach role, where he replaced
Sean Holley who immediately stood down as head coach of the region. Despite the sudden change at the Ospreys, Tandy led the Welsh region to the
2011/12 Pro 12 title, defeating
Leinster 31–30 in the final. He remained at the Region until 2018, guiding them to two more domestic semi-finals; losing out to Munster in both
2015 and
2017, and a European Challenge Cup quarter-final against
Stade Français in
2017. During his near six-year stint at the Ospreys he was in charge of 192 matches in all tournaments, winning 102 for a 53% success rate. In November 2018, Tandy was announced as the new defence coach for the
New South Wales Waratahs ahead of the
2019 Super Rugby season. During his first season, he was integral in significantly improving the Waratahs defence, where his defensive systems led to the media calling the Waratahs' defence the ‘blue wall’. His tenure with the Australian outfit lasted just one season, and by December 2019, Tandy left to join the
Scottish Rugby Union as defence coach for the
national mens side. In his first Six Nations campaign in 2020, Scotland had the best defensive record in the championship, and thet were joint top in 2021 and second best in 2024. Scotland's defence was integral in Scotland's significant victories over
Australia and
Argentina (twice), and wins over
England,
France and
Wales. He was also involved with Scotland during the
2023 Rugby World Cup. His efforts with Scotland's defence led to
Warren Gatland selecting Tandy as defence coach for the
British & Irish Lions during their
2021 tour to South Africa. In July 2025, he was announced as the new
Wales head coach, taking over on 1 September 2025. In doing so, he became the first Welsh-born head coach for the national team since
Gareth Jenkins was sacked from the position in 2007. He replaced
Matt Sherratt who acted as interim head coach from February to July 2025. Sherratt later joined Tandy's backroom staff as attack coach. ==Coaching statistics==