She previously held the portfolio for Local Government, the Welsh Language, Equalities and Planning in Plaid's shadow cabinet in the 5th Senedd.

[7] In 2018, she became co-deputy leader of Plaid Cymru alongside fellow Ynys Môn AM Rhun ap Iorwerth.

Upon her appointment by leader Adam Price, Siân Gwenllian stated that, "I'm proud to be a member of this group of talented and skilled individuals and look forward to working together as a collective as we move towards the project of building a New Wales".

[3] Siân was also appointed as an Senedd Commission Commissioner in November 2018 and was responsible for official languages and the delivery and transformation of services to Assembly Members.

[8][9] From 2018 to 2021, Gwenllian was Plaid Cymru's spokesperson for the Welsh Language, Education, Children and Young People and Culture.