The former entertainer and political novice recently starred in a popular TV series, "Servant of the People," in which he played a history teacher who is elected president after his rant about government corruption goes viral.
Zeleskiy declared victory Sunday night at his campaign headquarters.
"I'm not yet officially the president, but as a citizen of Ukraine, I can say to all countries in the post-Soviet Union look at us. Anything is possible!" he said.
Throughout his campaign, Zelenskiy has stressed that he's unmarred by political “filth,” and vowed to change the system.
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But his outlined policies were largely vague and short on detail.
Nevertheless, his message of change resonated with voters across the war-torn country with a flailing economy.
“It’s very important that this voting is based on reason,” said Poroshenko, while casting his vote accompanied by his grandson earlier Sunday. “It might be fun and hilarious at first, but I don’t want it to get painful later,” he added, in what appeared to be a reference to Zelenskiy’s comedy career.
The two clashed in a much-publicized debate at the nation’s largest sports arena Friday, hurling insults and attacking each other’s vision for Ukraine’s future.
Asked by the moderators to pose a "yes or no" question to each other, Zelenskiy didn’t hesitate, looking Poroshenko in the eye and saying: “Are you not ashamed?”
Poroshenko’s response, muted by loud cheering from the comedian’s supporters, was — no. “I am proud of Ukraine and the last five years,” the former president said.
But many Ukrainians don’t share that view. Despite his uncompromising efforts to keep the country on a pro-European path, Poroshenko has been accused of reluctance to tackle rampant corruption, a failure to reinvigorate the economy and an inability to end the war with pro-Russian separatists in the east — a five-year-long conflict that he promised to resolve in just two weeks before he swept to power in 2014.
“I am the result of your mistakes and promises,” Zelenskiy told Poroshenko during Friday night’s debate. “I am not your opponent,” he said, pointing a finger at the president. “I am your sentence.”
Zelenskiy has pledged to keep Ukraine on a pro-Western course, but has sounded less emphatic than Poroshenko about possible plans for the country of 42 million people to one day join the European Union and NATO.
“Zelenskiy’s presidency ushers in a period of major political realignment in Ukraine,” Dmitri Trenin, the director of the Carnegie Moscow Center think tank, told NBC News. He added that Zelenskiy will need to show from day one that he is “a radical break” from the past.
“I would expect Zelenskiy going after corrupt officials as a very popular move," he said.
There have been questions raised around what fate may befall Poroshenko after he loses the election — especially after Zelenskiy said that the former president might have to “meet with a prosecutor” during Friday’s debate.
Despite the loss, Poroshenko said he planned to stay in politics.
Kiev-based political analyst Valentyn Gladkykh told NBC News that Zelenskiy will now have to deal with a parliament, in which he does not have his own party and in which Poroshenko’s faction dominates.
Gladkykh says he will be hard-pressed to make friends with the lawmakers as he prepares for parliamentary elections in October, all while trying not to lose the support of his voters.
“Poroshenko, if he doesn’t do anything stupid after losing the election, will hand over power and start preparing for parliamentary elections, trying to destroy Zelenskiy’s approval rating, with the help of his faction in parliament and people in government," Gladkykh said.