Harry May, 21, of Luton, north of London, was fined £100 ($122) and ordered to pay £85 costs after pleading guilty to a public order offence.
The incident happened when Charles, 74, was in Luton on December 6 to meet community leaders and open a new Sikh temple.
Prosecutor Jason Seetal said the egg thrown by May landed near Charles as he chatted to locals. He said May told police he threw it because "he believed the king visiting a town like Luton, which is a deprived and poor area, was in bad taste".
"Whatever disagreement you have with somebody, the way to resolve it is not to throw projectiles at them," chief magistrate Paul Goldspring told May during a hearing at Westminster Magistrates Court.
The magistrate said he accepted that May was not trying to hit the king with the egg but said the attack was "planned" and "targeted" Charles.
The attack came less than a month after several eggs were thrown at Charles and his wife Camilla in the city of York, in northern England, in November.
None of the eggs hit Charles, and he was ushered away by minders.
Police detained the alleged attacker, 23-year-old university student Patrick Thelwell. He was later released on bail on condition that he did not carry eggs in public except when shopping. Thelwell will appear in court in York in northern England next Friday.
Charles became king on the death of his mother, Queen Elizabeth II, in September. She was buried after a state funeral and 10 days of national mourning.
But there were some protests against the hereditary principle of monarchy under which Charles took over as head of state.
This story has been published from a wire agency feed without modifications to the text.
