Seonaid shot at school (Dollar Academy) 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.

[6] 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.

[15] In August at the Rio de Janeiro leg of the World Cup McIntosh won Silver in the Women's 10m Air Rifle.

At the European Championships in September 2019 Seonaid won Bronze in the 50m Prone Rifle Mixed Team (Pairs) event with teammate Kenneth Parr.

[19][20] McIntosh was studying for an MSc Performance Psychology at the University of Edinburgh when the British teams were announced for the postponed 2020 Summer Olympics in Tokyo.

[23][24] In February 2023, McIntosh won the 10m Air Rifle event at the second stage of the 2023 ISSF World Cup in Cairo.

[25] This made her the first British athlete to win a World Cup Gold Medal in the air rifle event.

[32] At the 2024 ISSF World Cup in Cairo, McIntosh won gold in the 10m Air Rifle Mixed Pairs with teammate Dean Bale, scoring 10.9 on her final shot.