Mr Fletcher succeeds Martin Griffiths, also from the UK, who stepped down at the end of June for health reasons.
Tom Fletcher, UN's new chief of humanitarian affairs, assumed office on Monday, pledging to defend humanitarian values while navigating operations amid growing global crises.
Mr Fletcher, a former UK Ambassador and policy adviser to three British Prime Ministers, was appointed the United Nations (UN) Under-Secretary-General for Humanitarian Affairs and Emergency Relief Coordinator by Secretary-General António Guterres in October.
In his new role, he will also head the Office for the Coordination of Humanitarian Affairs (OCHA).
Mr Fletcher succeeds Martin Griffiths, also from the UK, who stepped down at the end of June for health reasons. In the interim, Joyce Msuya, the Deputy Emergency Relief Coordinator, served as acting Under-Secretary-General.
As the new top humanitarian official, Mr Fletcher acknowledged that he is "under no illusion as to how tough this mission is."
"The impact on civilians of conflict, climate change, inequality and injustice is growing at a staggering rate," he said in a statement.
"I will defend our humanitarian movement and humanitarian values, and battle to make us more efficient, strategic, inclusive and innovative. We must win afresh the argument for international solidarity," he added.
Before his appointment, Mr Fletcher was Principal of Hertford College, Oxford, and Vice Chair of Oxford University's Conference of Colleges.
He brings extensive experience in leading and transforming organisations, along with a strong background in diplomacy, having previously served as Global Strategy Director at the Global Business Coalition for Education, according to a UN biographical note.
Mr Fletcher also worked closely with the UN during his UK diplomatic career in Africa, the Middle East, and Europe, including as Head of the Middle East Peace Process at the Foreign and Commonwealth Office.
He is the 12th person to take this role since the creation of what is now the UN Office for the Coordination of Humanitarian Affairs (OCHA).
