He also vows to reopen an industrial park in the North Korean border town of Kaesong that was jointly run by the two Koreas before the government of impeached South Korean President Park Geun-hye, who is in jail awaiting a corruption trial, closed it past year following a nuclear test and long-range rocket launch by the North.
The vote will end months of leadership vacuum.
Park became the first democratically elected leader in South Korea to be removed from office, triggering a snap election to choose her successor.
Park is in jail, on trial, but denies any wrongdoing.
Moon was chief of staff for the last liberal President Roh Moo-hyun, who sought closer ties with North Korea by setting up large-scale aid shipments to the North and by working on now-stalled joint economic projects. Moon, a former human rights lawyer, has been criticized by his opponents for being a North Korea sympathizer, advocating for a dialogue between the two Koreas - a move which would likely ruffle Washington's feathers.
Former prosecutor Hong Joon-pyo was second with 23.3 percent of the votes and centrist candidate Ahn Cheol-soo had 21.1 percent. The early voting rate was 26.06 percent. Other presidential candidates were also seen voting early in the day.
South Korea's election body says almost 64 percent of the country's 42.4 million eligible voters cast their ballots as of 3 p.m. Tuesday, representing a faster pace than the previous presidential election in 2012. The nuclear issue is now central to inter-Korean relations.
Seoul Broadcasting System, which is responsible for putting their animation budget to good use like this, has a solid history of jazzing up its election cycles with pop culture-savvy overlays, previously turning to film franchises like Indiana Jones and The Lord of the Rings to ramp up interest in the results show.
The frustration many South Koreans feel with collusion and graft pushed Moon to victory.
Civil Rights Groups Vow to Battle Trump's 'Religious Liberty' Order in Court
Currently, under the Johnson Amendment , churches are not allowed to endorse or oppose a political candidate. Several prominent leaders of the religious left said it violated the USA separation of church and state.
The victor was expected to be sworn in on Wednesday after the release of the official result.
He is likely to quickly name a prime minister, who will need parliamentary approval, and main cabinet positions, including national security and finance ministers, which do not need parliamentary confirmation. "But it should be manageable if the U.S. can avoid shooting itself in the foot", said Scott Snyder, director of the program on U.S. -Korea policy at the Council on Foreign Relations. Pyongyang carried out a fifth test in September and is believed ready for another.
But analysts warn perceptions that the USA ignored South Korean input on its own security issues - compounded when Trump called both Chinese President Xi Jinping and Japanese Prime Minister Shinzo Abe to discuss North Korea, but the caretaker government in Seoul - have left a key relationship strained before it has even begun.
Following a standoff between President Donald Trump and North Korean dictator Kim Jong Un over Kim's vow to advance his country's nuclear weapons programs, Moon has talked more about bolstering national defense in what analysts see as an attempt to woo conservative voters.
"Cleanly eradicating the puppet conservative group that has committed intolerable crimes is the shortcut to new politics, new life and a new world", it said.
Moon was the clear front-runner in the two-month presidential race, which came after Park was impeached over leaking government secrets to a confidant.
"It remains a concern that the left of center, left-wing party in South Korea is going to do well", the official told Reuters, speaking on condition of anonymity. South Koreans voted Tuesday for a new president, with victory widely predicted for a liberal candidate who has pledged to improve ties with North Korea, re-examine a contentious US missile defense shield and push sweeping economic changes.
Park had decided not to vote, despite having the right to do so because she has not been convicted, media reported.