Ledecky, the youngest member of the American team, gave her rivals no chance as she led all the way to snatch the gold medal in a time of eight minutes 14.63 seconds, missing the world record in the longest event for women in the pool by half a second.
Spain's Mireia Belmonte Garcia took the silver, her second of the Games after she finished runner-up in the 200 butterfly two days ago, but was more than four seconds behind Ledecky, who slashed more than five seconds off her personal best.
Adlington held on to win the bronze despite fading over the last 150 metres after she had tried to take the field up to Ledecky, who set off at a cracking tempo and was under world record pace for all but the last lap.
Adlington ended Britain's long drought in swimming champions by winning two gold medals in Beijing four years ago but finished third in both the 400 and 800 this time.
Adlington's compatriot Fran Halsall then progressed into the final of the women's 50m freestyle, though fellow Brit Amy Smith narrowly missed out.The 22-year-old’s time posted a 24.63 seconds time and she confirmed she is well in the shake-up for the medals in the lottery nature of the event.
“It’s all to play for now,” she said. “I just needed to make sure I got in and I did that. I just need to go and rest up see what happens tomorrow now. I think we need a big one.”
Halsall was full of praise for her Team GB team-mate Adlington.
“I don’t know what she’s got to be disappointed about,” she said. “She’s like the most successful swimmer of all time.
“I’d chew off my right arm for a bronze medal. She’s just inspirational – the rest of the team think she’s unbelievable and what she did is amazing.
“Time is irrelevant, we saw that in the 100m. I thought it would take a lot quicker to win a medal but I’ve gone 24.1 seconds in this pool before and hopefully I can get back down and do it again.
“It’s going to be quick all eight girls are capable of swimming 24 lows. It’s literally close your eyes and hope for your best on the finish. I’m going to be pumped for it tomorrow – it’s got to be my turn at some point.”
Amy Smith, who qualified for the semi-finals via a dramatic swim-off, missed out on a place in the final by 0.016 seconds after clocking a time of 24.87 but she was happy to clock a time close to her best.
“I don’t know what it is about the 50m freestyle, it just doesn’t seem to like me very much,” she said.
“It’s a good time, similar to what I did in the swim-off this morning so I can’t really complain, it’s quite close to my best so that’s good.”