But now they barely speak, and he has subjected her to blistering public attacks. Why?
She has said to him, according to reports, that she is not going to hand him the presidency, and he will have to work for it.
"We're asking her -- the Unity Party is her party, it's the party that would bear her legacy and she ought to be supporting it," he said in an interview in February.
Where he sees a personal betrayal however, the rest of the world finds moral courage. Sirleaf is after all single-handedly responsible for keeping Liberia stable and unified for the past 12 years.
It is a role she has taken seriously enough that, at the beginning of the campaigns, she was forced to issue a stern warning to the 20 candidates jostling to replace her: "We hold them as political leaders who seek the highest office of our land to act with dignity and responsibility that befits that office — to live up to their commitments to ensure violence-free elections," she said.
It is of course a necessary ritual to note that Sirleaf, who is stepping down after two six-year terms in office and who has won the Nobel Peace Prize, is far from perfect.
Corruption remains a major problem -- the country ranks 90 out of 176 countries in the 2016 corruption perception index by Transparency International. Even Sirleaf had to admit this in an address to the Liberian congress this year.
Added to this are accusations of nepotism after the president appointed her three sons to major posts in the government, albeit they were qualified for those positions.
Many of its youth are unhappy and express hopelessness as to present and future economic opportunities.
According to the United Nations, young people constitute more than 60 percent of the population, and youth employment is nearly 90 percent.
Again, even Sirleaf has been forced to admit this, calling it a major threat to peace and security in a 2013 address.
A majority of the population also has no access to electricity, power cuts remaining frequent across the country and those with access have to pay either a premium or get a private diesel generator.
This is also a country that had only 50 doctors for its population of 4.3 million at the outset of the Ebola outbreak in 2014.
But then her government has substantially improved infrastructure, including a hydro-electricity dam, and she has been lauded globally for pushing investment into the economy.
Sirleaf negotiated a $4.7 billion debt relief and gained the trust of an international coalition including the World Health Organisation, the Food and Agriculture Organisation and UNICEF to fight Ebola.
However, there are two major achievements that stand taller than the developmental growth and challenges she has had to deal.
First is peace. Sirleaf - sometimes called The Iron Lady - has overseen a reign of uninterrupted peace in a country that has been ravaged by two civil wars.
Sirleaf has also consolidated democracy. This is the first time since 1944 that a democratically elected leader will hand over to another in Liberia.
She has allowed a boisterous democracy to take shape that has attracted 20 candidates vying for 2.2 million votes, and guaranteed a run-off in a race with no clear favorite.
Ultimately, she has made it very easy for Liberia to be seen as a serious democratic, developing nation; with a country in far better shape than when she took it over in 2005.
And the woman sometimes called Iron Lady, who has survived an abusive husband, "imprisonment, multiple brushes with death, and the vagaries of working with and against Liberia's various strongmen" has done all of this without an iron fist, a disrespect for rights or the assault on liberties that has been the hallmark of successive African 'liberators'.
Sirleaf leaves Liberia with a reputation intact, a nation on a path to growth, institutions set to thrive without her and an example that screams to the world that African leaders can say no to power, and yes to legacy. She leaves with her head held high.
One can only hope that Rwanda's Paul Kagame is paying attention.