McKay struck four times in the opening 30 minutes to put Inverness 4-0 up at the break and the striker then wrapped up the scoring late on after the visitors had added further goals through Josh Meekings and Liam Polworth as Caley remained unbeaten in pre-season.
Arbroath's consolation came from Darren Smith when they were 6-0 down.
Aberdeen suffered defeat in their penultimate pre-season fixture as they went down 3-2 at Peterhead.
Hosts Peterhead raced into a 3-0 lead through goals from former Dons Rory McAllister (24 pen) and Jordon Brown (33 and 36), and although Aberdeen pulled two back through Barry Robson (40) and Niall McGinn (73 pen), they could not find an equaliser.
Also tonight, Ross County went down 1-0 to Dutch side Den Bosch in a match which saw midfielder Richard Brittain sent off.