The Welshman, who was searching for a third Olympic medal after silver in Beijing and bronze in Athens, finished seventh in his heat in a time of 15:14.77 minutes.
The 27-year-old was disappointed to end in the manner he did but is content with a supremely successful career.
"I think that is probably is my last race,” he said. “I would have liked to have gone out on a better note.
“But swimming in your home country, having been in three Olympics, won two medals, a Commonwealth Games I would have taken it if you told me when I was a little 12-year-old boy starting out.
"It's been a privilege to swim in front of all these amazing fans. But at the Olympics, you only enjoy it if you get what you want, and I didn't want that result, obviously.
“It's been a tough year, a tough four years, my body is dealing with all the knocks you get as you get older, swimming in the distance events. But I'm proud that I persevered, I got here.
"I knew I wasn't in the best shape of my life, but I thought if I could use the experience and deliver something, I'd be in that final. Unfortunately I'm not.
“But my main goal is to have no regrets. In a few weeks, when the dust has settled, I'll be proud of what I've done. If this is my last race, it's not the way I wanted it to end, but I'm still a two-time Olympic medallist and that's more than I ever thought I'd achieve.
"You never make a decision when you're emotional. Retirement means nothing in sport these days, people are always coming back. I'll have a think about it, and whatever happens I'll be around supporting GB and Welsh sport for a long time.
"When you're in the moment, you want to get the best out of yourself and you lose sight of the bigger picture. Sometimes I forget how hard I've trained, for ten years doing what I do to my body every day is bound to have a payback.
"This time last year I was contemplating walking away, I was so frustrated, I took time out, came back and got the hunger for it, and I'm so glad I did because being here has been amazing.
“But I do this for success, and I'm disappointed that's not what I'm capable of, but it's not for want of trying."
However Dan Fogg, doubling up in both the 1500m and open water 10,000m at these Games, did qualify for tomorrow’s final.
The Games debutant swam a controlled race to post a time of 14.56.12 mins – just outside his own English record.
And the 24-year-old was happy to finally get in the pool after waiting nearly a week to compete in his first event.
“I am really happy to do that in the morning,” he said. “Being the first ones out we had to set a marker down.
"It is really hard having to wait all week but It is inspiring to see others, seeing Michael [Jamieson] win silver, it has given me so much energy that I didn't think I had."
Fran Halsall, fresh from a disappointing sixth in the 100m freestyle final last night, made the best of a slow start from a noisy start line to qualify third fastest for tonight’s 50m freestyle semi-finals.
The 22-year-old’s 24.61 seconds time leaves her full of confidence and she is determined to put herself well in the medal shake-up.
"If I go a bit faster I'll be happy,” she said. “I've got a bit of angry determination going on. I'm confident for this evening.
"When they said take your marks I kind of looked around. Hopefully, with a proper start, I'll get a faster result.
"I'm in a very steely and determined mood. I have nothing to lose right now. It's been like a rollercoaster of emotions. When I walked out I felt like a superstar.
"There are six girls who could get a medal and we're all very close (in time). If I get through to the final, I'm not going to let anyone beat me."
Team GB teammate Amy Smith had to survive a swim off to qualify for the semi-finals in her third swim of the morning after being part of the medley relay heats.
Both the men and women’s teams qualified for tomorrow’s 4x100m medley relay finals with impressive performances.
Liam Tancock, Craig Benson, Michael Rock and Adam Brown dominated their heat to win in 3:33.44 mins and move through second fastest behind the United States.
Gemma Spofforth, Siobhan-Marie O’Connor, Jemma Lowe and Smith swam 3:59.37 mins to qualify sixth for their relay final.