Ms McDonald and Ms O'Neill were selected unopposed to the two most senior leadership posts in the party.
She was appointed Sinn Fein's deputy leader in 2009 and has represented central Dublin in the Irish parliament since 2011.
Sinn Fein, which wants Northern Ireland to leave the United Kingdom and become part of the Republic of Ireland, is the second-biggest party in the Belfast assembly and the third in the Dublin parliament.
"There are many challenges facing the people of this island".
'Our focus must be on building Sinn Fein into an organisation that is fit for goal, and our objective is to win, to win elections, to increase our political strength, to realise our ambition of being in government north and south, to win progressive political victories every single day.
The outgoing Sinn Fein leader said however that he does not believe a united Ireland is inevitable.
"Sinn Fein in government, both North and South. Nobody should be asked to forgive if they can not do so", she said.
At that time he had also said Sinn Fein would be working towards a unity referendum to be held within the next five years. There is no value in engaging in the blame game.
The republican also heralded her appointment as a new dawn for the party which would bring "innovative and modern ways of advancing our politics". "We do not have to agree on the past, We must only agree that the past is never again repeated".
On Britain's upcoming departure from the European Union, or Brexit, she said Sinn Fein will not accept any deal that reinstates border controls between Northern Ireland and the Republic. She said that the talks regarding this are ongoing and Sinn Fein is committed to a positive income.
Regarding social issues, she said that Sinn Fein will campaign enthusiastically for the repeal of the Eighth Amendment of the Constitution of Ireland, which virtually bans abortions in the country.
On the Irish unity issue, Mary Lou McDonald said that "The war is over".
"We have made some progress but there are still considerable obstacles but as I said to our unionist friends this is the last chance agreement".
"There is now a peaceful and democratic path to Irish unity, a way to unite Orange and Green and end division".
Victims of the mainly Roman Catholic IRA included British soldiers, Northern Ireland police officers and Loyalist terrorist opponents, but also many civilian members of the mainly pro-British Protestant majority in Northern Ireland and numerous Catholic civilians who opposed the IRA or were accused of being informers or collaborators.