Halliday joined Boro in 2010 from Livingston as a winger, but he has since been moved into a more defensive role.
The 21-year-old started Boro's 3-0 win over Huddersfield and he earned praise from Mowbray who has decided to convert him.
"I never had any doubts about Andy Halliday. I was not sure he was going anywhere as an attacking player but I felt he could play left-back," said Mowbray.
"I thought it was a big ask to play him against Albert Adomah at the weekend but this was an opportunity for him to show us what he could do."