The new European Commission took power after being delayed one month.
European leaders marked ten years since the Treaty of Lisbon entered into force on the first official day of the new European Commission.
The new EU leaders made statements before European Parliament President David Sassoli and European Council President Charles Michel handed a copy of the treaty to the new Commission President.
Ursula von der Leyen is taking over the European Commission a month later than expected following the rejection of three commissioner-candidates by Parliament.
The Commission President tweeted a video on Sunday of her changing offices and moving into the Commission building in Brussels.
Treaty of Lisbon
The treaty entered into force on December 1, 2009, combining treaties on the European Union and reforming European institutions including the Council, the Commission and Parliament.
It clarified the union's operation and created a formal procedure to withdraw from the EU.
The changes outlined in the treaty gave the European Parliament a larger role, voting in the EU's executive body and increasing its legislative power.
"There could be no better day to begin the work than this anniversary... we are the custodian of the Lisbon spirit," von der Leyen said at the ceremony as she talked about the strength of the European Union.
"It is our duty to preserve this treasure," she said about Europe.
The new president of the European Central Bank, Christine Lagarde, spoke about the past ten years in Europe, stating that the decade marked incredible change including the rise of technology, a financial crisis, and the international community acting on climate change.
"It's to pass from an age of repair to an age of renovation and hope," Lagarde said.