The incident before the court took place on 6 December.
When Charles III visited the city of Luton, May threw an egg in his direction.
It did not hit the king, but the security service took him away from the crowd.
Harry May was later detained.
According to Sky News, 21-year-old May pleaded guilty to violating the law on public order during a meeting in London's Westminster Magistrates' Court.
The prosecutor at the trial said that May explained his act by saying that he considered the king's visit to "a city like Luton, which is located in a disadvantaged and poor area, evidence of bad taste."
According to him, by throwing an egg at the king, he wanted to "convey his point of view."
The court ruled that May was fined £100 and also paid £85 in court costs.
In November 2022, it was also reported that police detained a man who threw eggs at Charles III and Queen Consort Camilla while walking in the gate area of Micklegate Bar in York.