The Red Rose men can now look forward to taking control of Pool D in their remaining clashes against Chile and Samoa.
The ball ricocheted off prop Marler’s head for Lawes to scoop up and coast home, for England’s second – and crucial – score in Nice.
Brighton fan Marler then later revealed he had been practising heading the rugby ball in the early part of the day.
“You won’t believe this but back at the hotel, Dan Cole, Jamie George and me, as part of our warm-up, were practising headers and it came to fruition tonight,” he said.
Captain Lawes was the prime beneficiary of Marler using his loaf, with Lewis Ludlam claiming the first score and Freddie Steward and Joe Marchant also crossing.
England’s bonus-point win puts them in position to push towards the quarter-finals, and Lawes was left happy to reap the rewards of Marler’s unwitting header.
“It was quite a lucky try but I’ll take them how they come,” said Lawes. “It was a good bit of luck for us and in these conditions you take what you’re given.
“I saw it ricochet off Joe’s head, so I went to score the try just in case. But I was pretty sure that it had gone backwards and then come off his head.
“I told the ref that on the way back. I had a good idea that it was going to be given.”
The Brave Blossoms trailed England just 13-9 at half-time, only for England to pull away after the interval.
Japan boss Jamie Joseph was left to lament a match-turning moment on which his side had precious little influence in the shape of Lawes’ try.
“That’s a first for me, mate,” said Joseph. “We just didn’t get any luck at all today, in any situation. I guess that was a fair try. What more can you say really.”