Ellen Johnson Sirleaf became Africa's first female president in 2005, two years after the end of a brutal 14-year conflict.
She was re-elected in November 2011 in a poll marred by a low turn-out and a boycott by her main rival.
Winston Tubman pulled out of the second round of the presidential race, saying the vote had been rigged. He said he would not cooperate with Ms Johnson-Sirleaf's government, raising the prospect that her initiatives could be slowed in a hung parliament where her Unity Party failed to win a majority.
Ms Johnson Sirleaf has been accused by critics of having little to show for her first term, with alleged failures in the areas of anti-corruption, decentralisation and national reconciliation.
However, supporters say she deserves praise for ensuring stability and the rule of law, as well as for managing to gain international forgiveness of huge national debt, putting the country on a sound financial footing and making the impoverished country much more attractive to foreign investors.
The opposition has accused her of nepotism over the appointment of one of her sons as chairman of the national oil company. The son, Robert Sirleaf, in April 2012 began legal action against newspapers over the accusations, dismissing them as unsubstantiated. Another of her sons is deputy governor of the central bank.
She was awarded the Nobel peace prize in 2011 for what the prize committee said were her efforts to secure peace, promote economic and social development and strengthen the position of women.
Ms Johnson Sirleaf served as finance minister under President William Tolbert in the late 1970s and fled the country after the Tolbert government was overthrown. She has worked for the UN and the World Bank.
Some of the opposition to Ms Johnson Sirleaf stems from her one-time association with former Liberian leader Charles Taylor. She briefly supported the then warlord in his quest to overthrow military leader Samuel Doe.
She admitted to her initial support for Mr Taylor, saying he had misled her into believing the war was necessary for change to happen. However, she has so far ignored a Truth and Reconciliation Commission recommendation that she should not hold public office for 30 years for backing Charles Taylor.
Born in 1938, she is a widowed mother-of-four.