Most exit polls showed incumbent Petro Poroshenko in second place, meaning he would face Zelenskiy in a second round on April 21.
Combined with the popularity of his television show Servant of the People, in which he plays a teacher who becomes president, Zelenskiy's approach, heavy on humour, light on policy, has charmed an electorate jaded by corruption, war and two revolutions in just two decades.
Some have called it Populism 2.0 - like US President Donald Trump, Zelenskiy has run on the strength of his tough-talking on-screen character.
But rather than a border wall and trade tariffs, he only promises that "everything will be okay".
Many voters believe he's the only hope for change.
Since protests toppled a pro-Russian president in 2014, political and economic reforms have stalled, and the country has become the poorest in Europe. Corruption pervades, yet even Zelenskiy is not immune.
One of his companies was found to have made royalties and produced films in Russia during the war, and it was reported that he failed to declare a 15-room villa in an Italian resort favoured by Russian oligarchs like Roman Abramovich and Oleg Deripaska.
But in a country where statistically not even one in 10 people trusts the government, the candidate's pledge to run for a single five-year term to tackle corruption has found eager ears.
"The main thing is that he's not a politician, he's not caught up in dirty money," said teacher Nadezhda Solonets at a free show by his troupe at Dnipro Arena.
Zelenskiy has named Brazil's populist president Jair Bolsonaro as well as French leader Emmanuel Macron as his inspirations.
Although he has spoken of holding referendums, a tax amnesty and stripping officials of legal immunity, his programme remains ill-defined. One of his billboards reads "no promises, no apologies".
Analyst Volodymyr Yermolenko says: "Zelenskiy doesn't say he's going to cut gas prices or bring Ukraine into the European Union, he just says, 'Look at me, I'm a cool guy, I'm not a politician'."
It appears his supporters are voting less for him than for his character, Vasily Goloborodko, the everyman president he created for Servant of the People. But the political message is easy to see behind the laughs.