Seonaid shot at school and club level and did not harbour international ambitions until sister Jennifer's success at the 2010 Commonwealth Games and attendance at the 2012 Summer Olympics. Whilst a pupil at Dollar Academy she was a member of the school's winning 2013 team in the NRA's Ashburton Shield, as well as individually winning the School's Hundred that year. After beginning training in earnest in November 2012, she placed 11th in the Junior Women's 50 Metre Prone Rifle at the ISSF World Shooting Championships – her first major international. At the 2015 10M European Championships in Arnhem she won Silver in the Junior Women's 10 metre Air Rifle, making her the first British Junior in 35 years to medal at a European Championship, and setting a new British Under-21s Record. At the 2014 Commonwealth Games, she competed in the 10 metre air rifle event, placing 19th. In November 2017, Seonaid was selected for Scotland's team for the 2018 Commonwealth Games At the Games she qualified in the 10m Air Rifle in 7th place, placing 5th in the Final. In the 50m Prone Rifle she won the Bronze medal, and in the 50m Rifle Three Positions qualified 6th, working up to win the Bronze medal in the Final. As of 2016 was Seonaid enrolled on the British Shooting World Class Performance Programme, working towards Tokyo 2020. At the 2018 ISSF World Shooting Championships, McIntosh won the Women's Prone 50m Rifle event becoming World Champion, she went on to place fourth in the Women's 3x40 Rifle, earning a Quota Place for the 2020 Summer Olympics in Tokyo. In May 2019 McIntosh won a Silver medal at the Munich leg of the 2019 ISSF World Cup in the Women's 3x40 Rifle event, earning herself a place at the World Cup Final. She was not allocated an Olympic Quota Place however having already won one at the World Championships. This was the first Rifle World Cup Medal a GB shooter had won since Kenneth Parr won a Silver Medal for Prone in the 2016 ISSF World Cup, and the first Women's medal since Louise Minett won a Silver in 10m Air Rifle at the 1999 Atlanta World Cup. In August at the Rio de Janeiro leg of the World Cup McIntosh won Silver in the Women's 10m Air Rifle. She went on to win Gold in the Women's 3x40 Rifle, becoming the first British Woman to win a World Cup Gold Medal. This result moved her from fourth to first in the ISSF World Rankings for Three Position Rifle in the September 2019 Rankings, making her the first British Woman to be ranked World #1. At the European Championshipsin September 2019 Seonaid won Bronze in the 50m Prone Rifle Mixed Team event with teammate Kenneth Parr. She went on to win Gold in the 300m Prone Rifle, equalling the World Record with a score of 599.
