He was the Green Party's candidate in Hordaland constituency for the 2017 and 2021 election, but failed to secure a seat on both occasions.

[3][4] On 12 May 2018, he was elected spokesperson for the Green Party, alongside incumbent Une Bastholm, succeeding Rasmus Hansson.

[5] Following a national conference in 2019, the Green Party decided to elect a single leader, abolishing their tradition of having multiple spokespeople.

Shortly afterwards, Hermstad announced his leadership candidacy, while he in addition would become acting leader on 22 August until the autumn congress.

[8] He was formally elected at the extraordinary congress on 26 November, winning 102 votes against Kristoffer Robin Haug's 101.