As expected, seven-time gold medalist Ledecky dominated in the 400m freestyle championship before a crowd of 20,689 fans at Lucas Oil Stadium, home of the NFL's Indianapolis Colts.
"I never imagined I'd get to go to a fourth Olympic Games," Ledecky said.
The swimmer from Maryland said the joy of making the U.S. team never gets old and credited her coaches and teammates for her longevity in the sport.
"I feel like I enjoy this more and more each year and I think that's a testament to the people around me my whole career," she told reporters.
"Great communities that keep me excited about the sport, great teammates that push me every day, great coaches that believe in me and push me to reach for bigger and bigger goals.
"That's how I've been able to be consistent and I pride myself on that consistency."
Paige Madden was second with a personal best time of 4:02.08 but has not yet qualified for the Games.
Ledecky is widely considered the greatest female swimmer of all time and is just the eighth American swimmer to make at least four trips to an Olympic Games, where she will look to add to her tally of 10 Olympic medals.
Walsh kicked off night with a bang, setting a world record in the women's 100m butterfly to send a message to the world with just over a month to go before the Games.
Walsh led from the outset, made the turn at a world record pace and looked up in disbelief after she touched the wall in 55.18 seconds as the crowd erupted.
"I'm over the moon, just so happy," Walsh said.
"It's such a surreal experience. I'm trying to convince myself that just happened. I'm in awe."
Her performance in the semi-final heat beat the previous record of 55.48 seconds, which was set by Sweden's Sarah Sjostrom in 2016.
Indiana native Aaron Shackell punched his ticket to his first Olympics with a victory in the 400m freestyle.
"Ever since I learned my dad was an Olympian I always wanted to be an Olympian myself," said the 19-year-old Shackell, whose father Nick represented England in swimming at the 1996 Atlanta Games.
"It wasn't always easy. For a long time I wasn't even good at swimming and honestly didn't like it until a few years ago."
The trials will determine the U.S. team for Paris and are being held over nine consecutive nights in Indianapolis, which has transformed into a swimming paradise complete with a 66-foot replica of the Eiffel Tower located near the downtown stadium.
The throng of red, white and blue clad fans was the biggest to ever witness a swim meet, breaking the record of 16,000 set at the Rio Olympics in 2016.
