A time of eight minutes and 21.78 seconds put the Brit ahead of Lauren Boyle of New Zealand and secured her as fastest qualifier for the final.
And Adlington, world record holder in the 800m freestyle with 8:14.10 from Beijing, is not taking anything for granted as she bids to secure her fourth Olympic medal.
“I was ahead in the 400m heat and I only scraped through in eighth and I decided I’m not going to take chances and give it all I’ve got,” said Adlington.
“I’ve now got a day rest and I’ve just got to rest up as much as I can. I didn’t know what to expect after the 400m and then the heat and everything so I’m so pleased with that.”
Eleanor Faulkner’s time of 8:38.00 wasn’t enough to qualify for the final along with teammate Adlington, but the 19-year-old believes her London 2012 experience will make her stronger.
She said: “I just went out there to do the best I possibly could and it just wasn’t my day but I got in there and I think I raced it well.
“This is my first senior international and it’s just been an amazing experience to get out there and enjoy myself with the rest of the team.”
Steph Proud and Lizzie Simmonds secured their places in the 200m backstroke semi-finals in 11th and 14th place respectively.
Proud touched fourth in her heat with 2:10.01 to book her place in the semis along with teammate Simmonds, who swam 2:10.37, as both Brits progressed.
American Missy Franklin, who shared the pool with Proud for her heat, set the fastest time with 2:07.54 ahead of compatriot Elizabeth Beisel.
The 17-year-old from Colorado has already won two gold medals in the pool and has a great chance of winning more before the swimming competition ends on Saturday. The 200 backstroke is her strongest event after she won it at last year's world championship.
"That was my last prelim swim. It's kind of sad to think of it like that," said Franklin, who seems to have boundless energy and enthusiasm.
"I only have three more sessions left. I am just going to leave it all in those three sessions.
"There is a little bit of relief but I am going to miss this so much. Even though I am tired, I am trying to scoop up every last bit of energy that I have to get through these last three days."
Beisel was second fastest overall, 0.28 behind her team-mate, while Zimbabwe's Kirsty Coventry, who won the gold at the last two Olympics and is the current world record holder, was third.
"I am looking forward to getting some rest and try to recover as quickly as these youngsters I have to compete against tonight," the 28-year-old said.
"It's going to be tough to get into the final, but I am excited about that challenge."
The British men, however, had no fortune in their respective qualifiers.
Adam Brown finished seventh in his men's 50m freestyle heat, failing to reach the Olympic semi-finals.
Brown finished more than a second behind heat winner and fastest qualifier George Richard Bovell of Trinidad and Tobago to come 20th overall.
The final heat was won by world record holder Cesar Cielo with the second fastest time in a tough field ahead of fellow Brazilian Bruno Fratus, while James Magnussen of Australia also booked a lane in the semi-finals.
Michael Rock was second at the half-way point of his men's 100m butterfly heat, only to finish seventh in a tight race for an overall finish of 24th.
Antony James was eighth and last in his heat, won by Michael Phelps, with the American second fastest overall behind Chad le Clos. James was 31st overall.
Phelps, his powers seemingly diminishing by the day, provided a reminder of his incredible feats from Beijing when he beat his old rival Milorad Cavic.
It was only a morning heat but the American showed why he remains the fiercest competitor in the pool when he stormed past Cavic on the last lap to book his place in the semi-finals, his last individual event before he retires.
"This is my last prelim swim ever so that was pretty fun to be able to do a pretty decent time in it," Phelps said.
Phelps famously beat the Serbian by a fingernail in China four years ago to win the seventh of his eight gold medals and the pair remain on course to meet in Friday's final, as long as they get through Thursday night's semi-finals.
But Phelps, bidding to win the event for the third time, has a new and tougher challenger this time.
South Africa's le Clos beat Phelps with a desperate late lunge to win the 200 butterfly final and qualified ahead of him in the 100.
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