[8] Speirs entered the South Australian Parliament in March 2014, defeating Chloë Fox, Labor's Minister for Transport Services.

[9] In October 2014, Speirs was appointed to the Parliament's Economic and Finance Committee[10] following the resignation of Iain Evans.

[20] On 8 August 2024, Speirs stood down as Opposition leader, saying he had had a "gutful" of the job and wanted to give the party the best chance in the next election.

[21] Following his resignation from Opposition leadership, Speirs was embroiled in a scandal over a video that appeared to show him using illicit substances.

"[6] Speirs supported nuclear energy, a position that was backed by South Australian uranium miners.

[5] He faced criticism in 2021 when he spoke at a church and encouraged those in attendance to join the Liberal Party,[27] but later said the issue had been taken out of context.

[29] He is a founding member of the Hallett Cove-based environmental group, Friends of the Lower Field River,[30] a Landcare group established by Hallett Cove residents in 2006 to protect and care for the lower portion of the Field River and its environs.