Siersema has been active in Groningen politics since 2019. During her term as a member of the Provincial Council, she focused on nature and agriculture, water and climate adaptation, the Wadden Sea, finance, and economic affairs. Since 2023, she has been the parliamentary leader of GroenLinks. She was also a negotiator during the formation of the current coalition. Additionally, she worked in her hometown Stadskanaal as a teacher in special primary education.
With Siersemas appointment, the College of Deputies is back at full strength.