Hailing from Stonehaven in Aberdeenshire, Mr Johnstone worked as a dairy farmer before being elected to Holyrood in 1999. He served as his party's chief whip, as well as acting as spokesman for portfolios including housing, transport and rural affairs.
In the most recent session of parliament, Mr Johnstone was deputy convener of the finance committee and a member of the parliament's corporate body. He is survived by his wife Linda and their two children.
Ruth Davidson said:
"Alex's passing is an enormous loss for the Scottish Conservative party, for the Scottish Parliament, and for Scottish public life generally.
"He was a big man with a big heart. He embodied politics at its best: trenchant in his views, always up for a political fight, but respected and admired by all sides of the political divide for his decency and generosity"
Rest in Peace