[12][13][14] After progressing through the various youth team levels at Dundee United, Souttar signed a professional contract in July 2012 and became a member of the club's under-20 development squad.

[15] Souttar's form began to attract the attention of other clubs, with English Premier League team Sunderland having a bid of £600,000 accepted by Dundee United in October 2013.

[20] Souttar continued to play regularly in central defence throughout the 2013–14 season and scored his first goal for the club in a Scottish Premiership match against Aberdeen on 1 January 2014.

[22] After his return to the first team, Dundee United successfully experimented with Souttar in a defensive midfield role in matches against Celtic and Aberdeen during April 2015, a position in which he has subsequently featured regularly.

[25] On 1 February 2016, Souttar signed for fellow Scottish Premiership club Heart of Midlothian, agreeing a three-and-a-half-year contract for an undisclosed fee.

[28] He made a full recovery (though then missed three months in late 2018 with a hip injury which was aggravated on international duty),[29][30] and took part in the run to the 2019 Scottish Cup Final which Hearts lost to the same opponents.

[31] After missing much of the 2019–20 season with an ankle injury requiring an operation, Souttar returned to the team only to tear the achilles in his other leg during a Scottish Cup tie with Rangers in February 2020.

[35] On 14 January 2022, Rangers announced Souttar had signed a pre-contract agreement to bring him to the club at the end of the 2021–22 season once his contract with Hearts runs out.

[43] Alex McLeish called Souttar into the senior Scotland squad for the first time in May 2018,[44] but he was forced to withdraw due to hip and hamstring injuries.

[48] In November 2021, Souttar was called up by Steve Clarke to the Scotland squad for the 2022 FIFA World Cup qualifiers against Moldova and Denmark.