She ran for a seat in the Chamber of Deputies in the 2017 legislative election, as the third candidate in the Cambiemos list in Santa Fe.[3] The list was the most voted in the general election with 37.80% of the vote, and Lehmann was elected.[4][5]
As a national deputy, Lehmann formed part of the parliamentary commissions on Agriculture and Livestock, Co-operative Affairs, Industrial Law and Foreign Affairs, Industry and Production, and PyMes and Corporative Social Responsibility.[6] Lehmann was an opponent of the legalization of abortion in Argentina. She voted against the two Voluntary Interruption of Pregnancy bills that were debated by the Argentine Congress in 2018 and 2020.[7] During the 2020 debate, she tried to have the parliamentary debate shut down on religious grounds, as she claimed to represent the "majority of citizens" of the Argentine nation who are Roman Catholic.[8][9]
Ahead of the 2021 primary election, Lehmann was confirmed as one of the candidates in the "Vamos Juntos" list in Santa Fe.[10]Vamos Juntos lost in the Juntos por el Cambio primaries, and so Lehmann lost her chance at re-election. Her term expired on 9 December 2021.