The Brighton Phoenix sprinter was part of the mixed 4x400m relay team with teammates Lewis Davey, Emily Newnham, and Nicole Yeargin.
The quartet were expected to compete for a podium finish after qualifying second fastest in the morning heat but crossed the line in fifth place, 0.23s off bronze.
Teams are allowed to make one personnel change between the heats and final and rival nations grew notably strong, including the Netherlands who brought in superstar Femke Bol.
Harries ran the third leg and clocked an impressive 44.22s but said he was struggling towards the end, while Yeargin was unable to clinch a medal.
USA equalled the championship record to win gold, while Netherlands and Belgium completed the podium.
The 26-year-old is now focussing on the men’s 200m, his only individual event in Tokyo, with the heats starting on Tuesday.
“[The legs felt] horrible on the last 200m,” the former Brighton College pupil said, speaking after attending a Novuna-backed British camp – with the company financing the ambitions of millions across the UK, from helping business grow and individuals plan for the future, to backing British Athletics on the global stage.
“I mean I’m a 200m, 400m guy so if I’m going to overtake someone it’s in the first 200.
“I just had a lot of work to do, I got the baton in fifth and we’re here to win medals, we’re GB.
“Great Britain is here to win medals and not just compete and so I went out as hard as I could to try and hold it.
“I think, because of the rounds this morning, I just didn’t have that endgame.
“Still, it was a 44.2 split which was quicker than this morning and a very good split still, but I wish I could have just held that last 30 or 40 metres a bit more and dipped into the 43s and put Nicole in a better position to go and get us a bronze.”
Yeargin admitted she almost tripped up Dutch star Bol, who ran a controlled last leg.
“She knows that she can close hard so I think they just kept that steady speed and I should have trusted myself and overtaken her when I could,” she said.
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