Penn became the youngest member of the House of Lords when she joined the House on 21 October 2019:[9] she was succeeded as baby of the house by Lord Harlech following the election on 14 July 2021. She made her maiden speech on 30 January 2020 during a debate on Defence, Diplomacy and Development Policy.[10] From 29 October 2019 to 21 April 2020, she was a member of the Lord's Science and Technology Committee.[11] She served as a baroness-in-waiting, a junior government whip, from 19 March 2020 to 20 September 2022.[12] Between 30 October 2022 and 13 November 2023, she was a parliamentary secretary, the most junior level of minister, in HM Treasury.[11]