At 31 years of age, he is the youngest current senator and the second-youngest current parliamentarian (after Wyatt Roy). Elected as a City of Adelaide councillor in 2014, he resigned on 8 September 2015 following the resignation of Penny Wright from the Senate. Simms has worked in the community sector as a policy advocate, served on the boards of a number of community organisations including the Youth Affairs Council and Acquired Immune Deficiency Syndrome Council of South Australia, and worked briefly as a radio journalist.
He is also a freelance writer and resides in Adelaide.
He holds a Bachelor of Laws and Legal Practice (Honours), a Bachelor of Arts (majoring in politics and sociology) and a Graduate Certificate in Journalism. In 2008 he was admitted to the Supreme Court as a Barrister and Solicitor.
As of 2015 Simms is partway through a Doctor of Philosophy in political philosophy at Flinders University where he has also taught in the politics department. As of 2015, he is a co-convenor of the Société Anonyme Greens.