Hesselgren became the first woman to be elected into the Upper House of the Swedish Parliament after female suffrage was introduced in 1921.

She was the eldest daughter of a provincial medical doctor Gustaf Alfred Hesselgren and Maria Margareta Wærn.

Hesselgren had originally wished to be a physician, but her weak constitution had made her unfit for this profession.

She was management director of the Women's Work Environment Inspection from 1913 to 1934, and was also one of the founders of the magazine Tidevarvet which was launched in 1923.

Hesselgren was given the Illis Quorum award in 1918, and in 1921 she became one of the five first women to be elected to the Swedish Parliament after the introduction of women's suffrage alongside Nelly Thüring (Social Democrat), Agda Östlund (Social Democrat) Elisabeth Tamm (liberal) and Bertha Wellin (Conservative) in the Lower Chamber.

Kerstin Hesselgren, being the first of her gender in parliament, regarded herself to be the spokesperson of women in the Upper Chamber.

She also successfully intervened in the case of cartographer Olga Herlin who had been denied a state pension despite 37 years of service as Sweden's first female engraver.