The World Number 434 had birdies on the third and the eighth before adding further ones on the 11th, 12th and 14th in a blemish-free round on Porto Santo Island in the Portuguese archipelago.
"I'm delighted with that score. I've had a niggling hip injury so that's hindering my swing," said the Challenge Tour graduate.
"I've got no idea how I did it, I just woke up with it on Saturday morning and was struggling to walk. It certainly wasn't pretty today because I missed a lot of greens. I was just getting up and down very well, so I'm very happy to be five under.
"Of course it would be great to win this week. There are a lot of guys desperate to do that though so it will be tough. It's such a fine line between big money on The European Tour and playing EuroPro Tour. One minute you're playing main Tour but if you take your eye off the ball you're back in the small time."
Englishman Charlie Ford finished his opening round two shots back on three under after a 69 which featured an eagle on the tenth, birdies on the first and 15th and a bogey on the fifth.
But England's Chris Gane was on course to take the lead from McArthur as an eagle and five birdies had him leading on the course with three holes to play.
Gane started his round with a birdie on the tenth, his opening hole, before further birdies on the 12th, 18th, third and sixth, together with an eagle on the 14th and a bogey on the 13th left him leading on the course with three holes remaining.
Among the later starters were Scotland's Andrew Coltart and Richie Ramsay and England's Oliver Fisher.