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.
US proposes new plan to ease Israel-Lebanon tensions amid fighting
US Secretary of State Marco Rubio has spoke with both Lebanese President Joseph Aoun and Israeli Prime Minister Benjamin Netanyahu on the diplomatic negotiations between Israel and Lebanon and has proposed a plan to allow for "gradual de-escalation", a US official said on Sunday.
Netanyahu orders deeper Israeli incursion into Lebanon
Israeli Prime Minister Benjamin Netanyahu said on Sunday he had ordered troops to move further into Lebanon in the battle against Hezbollah, despite a ceasefire announced more than six weeks ago.
Ebola recoveries bring signs of hope in DRC as suspected cases emerge outside Africa
Four nurses who were being treated for Ebola caused by the Bundibugyo strain of the virus have been discharged from a hospital in Bunia in the Democratic Republic of Congo after recovering from the disease, the World Health Organisation said on Sunday.
Blast in Myanmar village reportedly kills 55 and injures dozens more
At least 55 people were reportedly killed in a blast in Myanmar's Kaung Tat village, which a rebel army said was caused by the accidental explosion of material stored for use in mining.
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