Ashley Gill-Web, who suffers from bipolar affective disorder, was found guilty by a London court of using threatening, abusive or disorderly behaviour.
District Judge William Ashworth said the 34-year-old Gill-Webb was suffering from a "manic episode" but was still in control of his actions.
"I am sure that he was weighing up the chances of being caught before throwing the bottle in an effort to disrupt the start of the race and put off Usain Bolt," Ashworth said.
"I am sure, therefore that he was at that point acting rationally and wrongly and that he intended to cause harassment, alarm or distress to the competitors."
The case was adjourned until Feb. 4 for sentencing, but Ashworth said he would limit the punishment to a community-based penalty.
Bolt, who won the gold medal in 9.63 seconds, said after the race that he hadn't noticed the incident.
Gill-Web, who lives in the northern city of Leeds, used an old ticket to get into the Olympic Stadium on Aug. 5 and pushed his way to the front of an exclusive seating area.
"Usain I want you to lose. Usain you are bad," he shouted, followed by an obscenity, and threw a plastic bottle onto the track behind the sprinters just seconds before the race started.
Dutch judoka Edith Bosch — who was sitting in the same section of the stadium — intervened and pushed Gill-Webb in the back before he was escorted out of the stadium and arrested by police.
Prosecutor David Robinson said Gill-Webb's actions were "reckless and irresponsible."
"This incident came close to disrupting the most-watched event of the 2012 Olympic Games, which was broadcast to millions of people across the world and for which many athletes had trained for years," Robinson said.
Gill-Webb, who has two previous convictions of criminal damage, has lost his job and received psychiatric treatment since the incident.