[5] He stood, unsuccessfully, as a Green candidate in the 1995 Perth and Kinross by-election, and for Edinburgh Pentlands in the 1997 United Kingdom general election, finishing seventh and sixth respectively.

In an emotional speech, he promised to be a critical voice on the environment in the newly created devolved Parliament.

[14] In 2004, he was a member of the Scottish Parliament team in the TV general knowledge show University Challenge: The Professionals.

[15] In January 2007, The Scotsman reported that Harper was being considered for the next Presiding Officer of the Scottish Parliament.

As part of the deal, fellow Green MSP Patrick Harvie was nominated to head the Holyrood Transport, Infrastructure and Climate Change Committee.

[25] In August 2021, Harper criticised the Bute House Agreement that the Greens had struck with Nicola Sturgeon's government, claiming that his party had failed to take tougher action on North Sea oil, marine protection and taxation.

[26] In September 2022, he joined the board of Gordon Brown's unionist think tank, Our Scottish Future.

[29][30] In June 2024, he joined the Labour Party citing their plan for combatting climate change and their ability to beat the Conservatives in the 2024 United Kingdom general election.