Although the campaign was unsuccessful, Johnstone attended a Green party conference as a non-member and became more involved in politics.

She served alongside Robin Harper, and both were replaced in 2008 as co-conveners by Patrick Harvie and Eleanor Scott.

[9] On 22 May, she was announced as the Scottish Greens spokesperson on Health and Sport, Social Security, Children and Young People.

[11] In 2020, she introduced a measure to grant protected species status to the mountain hare in Scotland, which was accepted on 17 June 2020.

[15] The Presiding Officer is expected to be strictly nonpartisan, so Johnstone resigned her Greens membership to take on the role.

[15] On 14 May 2021, SNP MSP Annabelle Ewing and Liberal Democrat Liam McArthur were elected as the two Deputy Presiding Officers.

[18] This is the first time since the establishment of the Scottish Parliament in 1999 that neither the Presiding Officer nor any of the deputies has been a Labour or Conservative MSP.

[18] On 16 March 2022, Johnstone was appointed to the Privy Council by Her Majesty The Queen and therefore granted the title 'The Right Honourable'.

[19] She said in response to her appointment: Johnstone said: “It was a great privilege to be elected Presiding Officer of the Scottish Parliament last May, and it was an honour to be made a Privy Counsellor by Her Majesty The Queen this afternoon.