The Dumfriesshire, Clydesdale and Tweeddale MP has joined his son – Scottish MSP Oliver Mundell – in backing Ms Truss.
The foreign secretary is a clear frontrunner over challenger Rishi Sunak to replace Prime Minister Boris Johnson, with a YouGov poll released on Tuesday showing Ms Truss holding 69 per cent of the headline vote amongst Conservative members.
Writing in the Scottish Daily Mail, Mr Mundell stressed Ms Truss would be “steely” in dealing with First Minister Nicola Sturgeon.
His comments come after Ms Truss on Monday described the SNP leader as an “attention seeker” during a hustings event in Exeter.
Mr Mundell wrote: “I know the strains our precious Union has come under due to those vying to see it broken up.
“Almost a decade ago, Scots voted decisively to stay part of our Union.
“Yet the Nationalists are still desperate to refight that referendum in a shameless excuse to get away from their record of delivery.
“We need a leader who has the courage of their convictions as a Unionist.
“They need to be steely in order to work with Nicola Sturgeon, rising above her petty nationalist games and grievances. They need to be dedicated to delivering for every part of our United Kingdom, no matter the challenge.
“That is why I am backing Liz Truss. I know how deeply and personally – from her own life experience as a ‘child of the Union’, growing up in Paisley and Leeds – she feels that unique connection we have across our United Kingdom.”