Former Scotland midfielder McKinlay has been out of work since being sacked by Championship side Watford after only two games in charge.
The 45-year-old will now link up with new Sociedad boss Moyes, who took over in San Sebastian earlier this month and secured a goalless draw in his first game in charge against Deportivo La Coruna last weekend.
McKinlay was employed as assistant coach of Northern Ireland before taking over as Watford manager.
He also spent time on the coaching staff at Fulham and was appointed as joint caretaker manager of the London club before Roy Hodgson took over at Craven Cottage.