The education and lifelong learning spokeswoman and deputy convener of the Public Audit Committee said she was retiring to spend more time with her family.
She was elected in 1999, 2003, 2007 and 2011. She spent a year out of parliament to stand in the Moray by-election, following the death of Margaret Ewing. Her move to stand down follows similar decisions by other north MSPs, Rob Gibson and David Thompson of the SNP.
She said: “It was a great honour to be one of the 1999 intakes of MSPs. This was a historic occasion and a great privilege to be part of it.
“I am proud of my working class background. My father was a farm worker and instilled in us an ethic of hard work. I deeply believe that if I can be elected as an MSP from my background, having left school at 15, becoming a single parent with two children under three years old, then anyone in Scotland can do the same.
“Watching my grand-daughter’s chemotherapy treatment for leukaemia over the last two years brought home the precious time we have with our families as they grow up. Alba is now eight and was diagnosed with leukaemia at six.”
Scottish Conservative leader Ruth Davidson said: “Mary has been a fantastic servant to the parliament and the wider Conservative party. She’s a real grafter and a fearless champion of the Highlands and of her beliefs.”