She was third on the party list and was comfortably elected as one of the 18 Greens deputies, becoming deputy leader of the party's parliamentary group in the new Landtag. She served as spokesperson for women, integration, migration, and refugee policy. From 2014 to 2016, Spiegel also served on the Speyer city council. In the 2016 state election, Spiegel was again the Greens' candidate in Speyer, this time winning 10.4% of votes. She was again elected third on the party list. After the Greens entered a coalition government with the Social Democratic Party and Free Democratic Party, Spiegel became Minister for Family, Women, Youth, Integration and Consumer Protection in the second Dreyer cabinet. She resigned from the Landtag to join the cabinet and was succeeded by Pia Schellhammer. As one of the state's representatives at the Bundesrat, Spiegel is a member of the Committee on Agricultural Pol­i­cy and Consumer Protection, the Committee on the Environment, Nature Conservation and Nuclear Safety, the Committee on Economic Affairs, and the Committee on Transport. She is also a member of the German–French Friendship Group set up by the German Bundesrat and the French Senate. As State Minister for Families, Spiegel continued the work of her predecessor Irene Alt in advocating for marriage equality, campaigning for a vote in the Bundestag on the issue. In November 2016, she wrote a protest letter from the Bundesrat to Norbert Lammert, then President of the Bundestag. The initiative undertaken by Spiegel led to the introduction and passage of the Act introducing the right of marriage to persons of the same sex in June 2017. Spiegel became the first minister in Rhineland-Palatinate to take maternity leave, which she did in April 2018 to give birth to her fourth child. Spiegel brought the baby to a meeting of the Federal Council in October 2018. In November 2018, Spiegel represented the German federal government at the Women MPs Conference on the subject of "100 years of women's suffrage" in London. There she also met then-Prime Minister of the United Kingdom Theresa May. At the meeting, Spiegel urged her to prevent Brexit. In the 2021 state elections, Spiegel was nominated as lead candidate for the Greens. The party won 9.3% of votes in the election, becoming the third-largest in the Landtag. The coalition government between the SPD, Greens, and FDP was subsequently renewed, and Spiegel became Deputy Minister-President and Minister for Climate