The down-to-earth 23-year-old held off Canadian teenager Summer McIntosh and American great Ledecky to join Dawn Fraser as the only Australian female swimmer to retain an Olympic crown.
The world record holder was in full control to touch in 3mins 57.49secs ahead of McIntosh (3:58.37) and Ledecky (4:00.86), who faded badly in the final 50m.
"I can't believe that's me, to be honest," she said when asked about what the feat meant.
"I look at myself and I'm so normal. I love swimming, and I love getting out and representing my country and having fun.
"I hope nobody looks at me any differently. I'm just the same old goofy Tassie (Tasmanian) girl out here living out her dream.
"I hope it goes to show, anyone can do what they want to do if they work hard and believe in themselves. And here I am, from little old Launie (Launceston), a town of 90,000, and I'm out here living the dream."
Titmus, who dethroned Ledecky in Tokyo, came to France in red-hot form after shattering the 200m world record and swimming the second-quickest 400m ever at the Australian trials.
But Ledecky can never be written off and McIntosh was always going to be a threat after snatching Titmus's world record last year before the Australian took it back.
However, it was all Titmus in Paris, who was near world record pace at the halfway mark and comfortably surged home, with McIntosh winning her first Olympic medal by taking silver.
"I'm just happy to get the result for myself, and I'm so honoured to be part of the race and be alongside legends like Katie," Titmus said.
"I look up to her so much as an athlete, and it is certainly not a rivalry beyond the races. I really respect her as a person. Her longevity in the sport. It's nice to catch up and have a chat."
"It's fun racing the best in the world," she added. "It gets the best out of me; it gets the best out of them. I really hope all the hype lived up to the expectation. I really hope that I put on a good show tonight and everyone enjoyed it."
Titmus will also swim the 200m and 800m in Paris.
Ledecky has opted out of the shorter distance, focusing instead on defending her 800m and 1500m titles.
McIntosh is also skipping the 200m free, instead prioritising the 200m and 400m medley and 200m butterfly.
