[2] Thomson took up running in her early thirties in order to get fit and lose weight after the birth of her third daughter in 1989.

She was coached for some time by John Linnaker, the former Scottish steeplechase record holder, and began to concentrate on the marathon and shorter distances.

[9] In 1995, Thomson was selected to represent Great Britain in the IAAF World Cup Marathon in Athens alongside another Scottish runner, Alison Rose.

Her 25th place qualified her for selection for the 5th IAAF World Championships held in Gothenburg, Sweden, in August 1995.

She was invited to run marathons in many countries including at Sliema in Malta (1994), Walt Disney World in Florida (1995), Las Vegas (1996), Singapore (1996), Dubai (2000, 2001), Stockholm (2000, 2002), Pyongyang, North Korea (2001), Seville (2002), Beirut (2003) and Mumbai (2004).

Between 1993 and 1995, she won several Scottish half marathons: Glen Clova three times, Inverness and St Andrews.

[13] She was selected to represent Great Britain at the IAAF 1995 World Championships Half Marathon, held on 1 October in Montbéliard, France.

[16] In 1999, she was selected to represent Scotland in the World Mountain Running Trophy in Mount Kinabalu Park, Sabah.

[18] In April 1993, Thomson ran her first 100-kilometre race at Greenwich, completing the distance in 8:36:53, a new Scottish track record.

[20] On the strength of these performances she was selected to represent Great Britain at the IAU 100 km World Championships at Torhout in Belgium on 7–8 August 1993.

Thomson finished ninth in 8:12:05 and was part of the silver medal–winning team along with Carolyn Hunter-Rowe (1st) and Hilary Walker (5th).

[9] On 2 April 1994, Thomson competed for the first time in the Two Oceans Marathon, a 56-kilometre road race in South Africa.