Thomson joined the Scottish National Party at the age of sixteen in 1981. She became prominent politically during the Scottish independence referendum campaign, with the "Yes" campaign. She was elected as the Member of Parliament for Edinburgh West in 2015 with a majority of 3,210 votes and a 39% share of the vote; with a swing of 25.8% from Liberal Democrat to SNP. "The Sunday Post" described her as one of the SNP MPs to watch in the new parliament. "The National" described Thomson as "a breath of fresh air" in light of her broad-based life experience.