The 23-year-old dominated her race from start to finish but then had to look on as two extremely fast heats threatened to edge her out of the final.
However, her time of 4:05.75 seconds just saw Adlington through, although the event has such strength-in-depth that making the podium will be a battle, let alone successfully defending her title.
World record holder and world champion Federica Pellegrini was only seventh with Frenchwoman Camille Muffat quickest in 4:03.29.
Beijing 2008 bronze medallist Jo Jackson was 21st in 4:11.50.
Allison Schmitt of the United States became the third fastest in the world in 2012 at last month's USA trials and today was just 0.02 seconds behind Muffat.
Sun Yang and Ryan Lochte returned to the pool after being crowned Olympic champions in the 400m Freestyle and 400m Individual Medley respectively, with the pair qualifying fastest in the 200m Freestyle heats.
Matt Grevers headed the 100m Backstroke heats while James Magnussen anchored the Australia sprint freestyle squad into tonight's final as fastest qualifiers ahead of the USA.
Emily Seebohm set an Olympic record of 58.23 in the 100m Backstroke while 15-year-old Lithuanian Ruta Meilutyte headed the 100m Breaststroke in a stunning 1:05.56.