O'Neill was forced to resign after losing the support of a majority of members of parliament.
Over the past month, Marape has led the defections from the Government to the opposition side, but after O'Neill's formal resignation he brought his supporters back to the ruling coalition, the Australian Broadcasting Corporation reported.
Marape, 48, has been an MP July 2007, representing the electorate of Tari-Pori Open.
Marape attended Minj Primary School and Kabiufa Adventist Secondary School. He graduated with a BA from the University of Papua New Guinea in 1993, and a postgraduate Honours Degree in Environmental Science in 2000. He was acting assistant secretary of policy with the Department of Personnel Management from 2001 to 2006.
Marape first contested the Tari-Pori seat at the 2002 election for the People's Progress Party, when voting in the Southern Highlands Province was cancelled due to widespread violence. He contested the supplementary election in 2003, but lost to incumbent MP Tom Tomiape in a contest marred by the bashing of a polling official by his supporters.
He challenged the result in the Court of Disputed Returns, but both his initial petition and a subsequent appeal were rejected.
He contested the seat for a second time at the 2007 election as a National Alliance candidate and defeated Tomiape.
He was subsequently appointed parliamentary secretary for Works, Transport and Civil Aviation by Prime Minister Michael Somare.
He was also given additional responsibilities as Deputy Chair of the Privileges Committee and member of the Parliamentary Referral Committee on Inter-Government Relations.
He was Minister for Education from December 16, 2008, to August 2, 2011. In February 2012, he left the National Alliance Party and joined PNC.
He was re-elected at the 2012 election in the Tari-Pori District. He was then appointed as Finance Minister under the O'Neill government.
He was re-elected at the 2017 election, representing PNC.
On April 11, 2019, he resigned as Minister for Finance, but remained a member of PNC and the government. However, he resigned from the party on April 29, 2019. Sam Basil was appointed as Minister for Finance on April 18, 2019.