It follows reports that President Trump was outraged by comments made by Mulvaney during the Ukraine corruption saga.
The allegations of abuse of power over the Ukraine incident led to impeachment proceedings against President Trump.
Mulvaney's departure has been on the cards for the last few weeks.
Mr Mulvaney is to become US special envoy to Northern Ireland, while North Carolina lawmaker Mark Meadows has been appointed to take over his former position.
Powerful winter storm shuts schools, disrupts travel across US Northeast
Children across parts of the US Northeast will stay home on Monday as a powerful winter storm forced school closures and pushed offices and transit systems onto emergency schedules, with officials across the region warning of heavy snow, strong winds, and dangerous travel conditions.
Mexican military kills cartel boss 'El Mencho' in US-backed raid
One of Mexico's most notorious drug lords, Nemesio Oseguera, or "El Mencho", has been killed in a military raid on Sunday, sparking widespread retaliatory violence.
Afghanistan says Pakistan strikes kill and injure dozens
Pakistan said it launched strikes on targets in Afghanistan after blaming recent suicide bombings, including assaults during the holy month of Ramadan, on fighters it said were operating from its neighbour's territory.
Police officer killed, dozens injured in bomb explosions in Ukraine's Lviv
One police officer was killed and 24 other people were injured after several explosive devices detonated at midnight in Lviv in western Ukraine, the National Police said on Sunday.
Trump pivots to new 15% global tariff after Supreme Court setback
President Donald Trump said on Saturday he will raise a temporary tariff from 10 per cent to 15 per cent on US imports from all countries, the maximum level allowed under the law, after the US Supreme Court struck down his previous tariff programme. The move came less than 24 hours after Trump announced a 10% across-the-board tariff on Friday after the court's decision. The ruling found the president had exceeded his authority when he imposed an array of higher rates under an economic emergency law. The new levies are grounded in a separate but untested law, known as Section 122, that al
