Ashton ended day one on 4,661 points, 220 ahead of compatriot Trey Hardee. The 24-year-old broke Czech Roman Sebrle's 11-year world record of 9,039 points at the US trials, where at the halfway stage in Eugene, Oregon he had earned 4,728 points.
"I felt everything was solid. Some things I'm happy with, some things I'm not but it was decent enough," Eaton said of his day's efforts.
Victory in the decathlon brings with it the unofficial title of the world's greatest athlete but the toils of Eaton and his colleagues were a sideshow in the Olympic stadium for much of Wednesday.
Eaton ended a long day that had started with the 100 metres just after 1000 local time with the fastest time in the 400 of 46.90.
He cleared 2.05 in a high jump competition that got underway an hour earlier than the rest of the evening athletics programme.
His three failures at 2.08 were conducted against the noise for the semi-finals of the 110 hurdles which drew the crowd's attention.
A few hours earlier, Eaton had been grateful for the support of those who stayed to watch the conclusion of the shot put long after the track events were over.
"The fans don't really understand the decathlon, but from the crowd out there they're learning," Eaton said. "To stay for the shot put, which is not that exciting proves that."
The American got his first Games off to a flying start with the fastest 100 metres in an Olympic decathlon of 10.35 seconds.
Hardee was the second fastest with 10.42.
Eaton won the long jump with 8.03 metres before going close to his personal best in the shot put with 14.66 to finish 10th behind Kazakh Dmitry Karpov (16.47).
"It was a good start," Eaton said of his time in the 100. "I won and that is what I wanted to do."
Canada's Damian Warner lies in third place with 4,386 and Ukraine's Oleksiy Kasyanov is fourth on 4,346.
Sebrle's fourth Olympic Games ended in bitter disappointment when the Czech Republic champion from 2004 pulled out of the competition after the 100 metres after failing to shake off with a heel injury.
The decathlon continues on Thursday with the 110 hurdles, discus, pole vault, javelin and the concluding 1,500.