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.
At least 14 killed in explosion at weapons depot in Syria
At least 14 people were killed and 11 wounded on Wednesday in an explosion at a temporary site used to store weapons and remnants of war near the Syrian city of Sarmada in Idlib province, state television reported.
US hits five Iranian oil tankers after attack on Navy ships
The United States has destroyed five Iranian oil carriers on Tuesday after the Islamic Revolutionary Guard Corps (IRGC) targeted its navy warship with ballistic missiles "twice over the past two days".
UK gives air traffic provider a week to probe outage that halted flights
Britain's NATS air traffic control provider has a week to investigate the systems failure that halted flights and stranded hundreds of thousands of passengers, the government said, as pressure mounts on its chief executive.
Serbia's President Vucic dissolves parliament, calls snap elections
Serbia's President Aleksandar Vucic on Wednesday dissolved parliament and announced snap elections for next month as he seeks to extend his hold on power by running for prime minister.
With large crowds attending, Norway lays King Harald to rest
Norway's King Harald Vwas being laid to rest on Wednesday, in a series of ceremonies marking the culmination of 13 days of national mourning following his death at the age of 89.
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