Johnson, who came third in Liberia's presidential polls, said he decided to go with the ruling Unity Party as the main opposition Congress for Democratic Change wanted to set up a war crimes tribunal.
Johnson said he did not want to go to The Hague where the International Criminal Court is based and preferred to deal with Sirleaf who is also indicted by a Liberian Truth and Reconciliation Commission, TRC, based on the South African model.
Sirleaf has failed to implement the recommendations of a TRC report which named her on a list of people who should be barred from office for supporting warlord-turned-president Charles Taylor.
Sirleaf, who made history in 2005 when she became Africa’s first elected female president, said she initially backed Taylor’s efforts to overthrow dictator Samuel Doe, but became his fierce opponent when the full extent of his atrocities became known.
Prince Johnson was also named in the TRC report among those who should face prosecution. Johnson was infamously filmed ordering his men to cut off the ears of dictator Samuel Doe.
He has emerged as a surprise kingmaker after 11 October first-round elections, which Sirleaf won with 44 per cent of the vote. However she did not manage to win an absolute majority and heads to a run-off election on 8 November.
Her re-election bid could receive a significant boost from Johnson's 11.8 per cent of the vote.
Despite his violent past, Johnson retains huge support in the populous and iron-rich Nimba country where he was born.
Although he has given his support to Sirleaf, he says he is "adaptable” and “can change at any time."
Sirleaf's main rival Winston Tubman scored 31 per cent, and has accused the elections commission of rigging the vote, raising tensions in the country whose peace is still fragile eight years after the end of a brutal civil war.