Mr Jack was previously a backer of Boris Johnson and remained in post while many of his colleagues resigned over the Chris Pincher scandal.
He remained neutral throughout the Tory leadership election, but later added he regretted Mr Johnson had been forced out of office.
The MP for Dumfries and Galloway said he would focus on “growing the economy”, adding: “Liz Truss has been clear she will deliver for people across the UK.
“I look forward to being part of her new cabinet as we tackle the challenges facing our country, not least the rising cost of living and ensuring the long term security of our energy supply.
“We are committed to supporting families and business in Scotland and to strengthening Scotland’s place as part of a vibrant and successful United Kingdom.”
Mr Jack joins colleagues such as Kwasi Kwarteng who has been appointed as Chancellor and Robert Buckland who is Mr Jack’s counterpart in the Wales Office.