In the first ever Scottish Parliament election, Marwick ran as an SNP candidate for the Central Fife constituency, but came second to Henry McLeish.

Marwick contested Central Fife again in 2007 and was elected as the constituencies MSP after defeating the Labour incumbent, Christine May, with a majority of 1,166 votes.

In October 2012, Marwick accepted a nomination to join the Privy Council, meaning that she is styled the Rt Hon.

[11] Marwick was announced as the new chair for the NHS Fife board in November 2016, replacing Allan Burns who resigned from the post in August 2016.

[14] In 2022 a student newspaper at St Andrew's University ran a satirical article in which it criticised Nicola Sturgeon as "too scary" for Hell and suggested that England “boot Scotland out of the union”.

Marwick denounced St Andrew's as “not a Fife university” and described students as “pathetic wee trolls”.

“I regret the Saint run by the University of St Andrews students just buys into every stereotype we Fifers have of them," she tweeted.

It was therefore wholly unacceptable for the chair of NHS Fife to trash the whole university with offensive remarks too.” Marwick later deleted the tweet.