Moon's presidency began even before an inauguration ceremony as his election was a result of the rare by-election that sought to fill the top elected office vacated by former President Park Geun-hye, source from the Yonhap.
The former conservative leader was removed on March 10 by a Constitutional Court ruling that upheld her parliamentary impeachment late last year over a series of corruption allegations that have also led to Park's arrest and indictment.
Moon's five-year term began at 8:09 a.m. Wednesday when the National Election Commission (NEC) confirmed his victory in the election held Tuesday, the commission said. The liberal candidate from the Democratic Party secured 13,423,800 votes, or 41.08 percent of the total 32,807,908 votes cast, according to the NEC.
A written certificate confirming Moon's election was presented to the new president. The certificate was accepted by Rep. Ahn Gyu-back of the Democratic Party on his behalf.
Moon's first order of business came from his role as the commander in chief.
"President Moon Jae-in began his official term as the 19th president as he exercised his legal authority as the commander in chief at 8:09 a.m. on May 10," the presidential office said in a press release.
The office said Moon held a telephone conversation with Lee Sun-jin, Army general and chairman of the Joint Chiefs of Staff, and was briefed on the country's current security conditions and possible military movements by communist North Korea.
He then visited Seoul's national cemetery to pay his respects to his late predecessors and war heroes.
He was scheduled to meet with the leaders of all five parliamentary parties, starting with Rep. Chung Woo-taik, floor leader and interim chief of the conservative Liberty Korea Party, which has now become the main opposition party as the result of the election.
The president was expected to ask for the political leaders and their parties' support and cooperation for his new government, according to his aides.
Following his meetings, Moon was set to attend a scaled-down inauguration ceremony at the National Assembly.
Then after that, he is scheduled to hold his first press conference as president at the presidential office of Cheong Wa Dae to announce his nominees for four key government and Cheong Wa Dae posts, including prime minister and chief of staff.
Sources said Moon will likely tap Lee Nak-yon, the incumbent governor of South Jeolla Province, as the new prime minister and Im Jong-seok, a former lawmaker and his top secretary throughout his election campaign, as his first chief of staff.
Officials at the presidential office noted that Lee, following his nomination, may help find nominees for other government posts, with Im taking charge of finding new presidential secretaries.