Voters Sunday gave President Volodymyr Zelensky and his new political party the power to reshape Ukraine’s economic and political systems. Now Ukrainians will find out if the newcomers they’ve gambled on are up to the challenge.

Servant of the People—a party named after a Ukrainian comedy series in which Mr. Zelensky portrays the president—finished first in parliamentary elections with about 42% as votes are still being counted. This will almost certainly be enough for an outright parliamentary majority under Ukraine’s electoral rules. The opposition will include a pro-Russian party, along with groups led by former Prime Minister Yulia Tymoshenko and former President Petro Poroshenko.

Mr. Zelensky called early elections shortly after winning Ukraine’s April presidential election by nearly 50 points. His mostly substance-free presidential campaign focused on the country’s endemic corruption and ending the five-year war with Russia in Eastern Ukraine. The parliamentary elections followed similar themes: The party’s candidate list was filled with political newcomers, and Mr. Zelensky has promised to empower technocrats.

Power over domestic politics is concentrated in parliament, but the president oversees defense and foreign policy. Despite little knowledge of international affairs, the new president kept his cool after Vladimir Putin offered Russian passports to Ukrainian citizens in contested regions. He’ll have to keep a steady spine as Mr. Putin tests him ahead of possible negotiations.

Mr. Zelensky will also have to negotiate with the International Monetary Fund, but domestic reform is more consequential. Voters took a flyer on the unknown because the country’s political class squandered reform opportunities after the 2013-2014 Euromaidan revolution.