She used the occasion to highlight the plight of mothers and women in Ethiopia, urging the lawmakers to work towards building a society that rejects the oppression of women.
During her diplomatic career, Zewde was also head of the United Nations office to Kenya and has acted as Ethiopia's ambassador to several countries including France, Djibouti and Senegal.
Sahle-Work Zewde has been elected Ethiopian president by members of parliament.
The influential Addis Standard cited top government officials as confirming that Ambassador Sahle-Work Zewde, a top official with the United Nations, UN, was due to be Mulatu's replacement.
"When there is no peace in the country, mothers will be frustrated", Sahle-Work said before Parliament shortly after her approval.
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The prime minister, who is the head of state, holds executive power.
After being sworn in, President Sahle-Work promised to work hard to make gender equality a reality in Ethiopia.
Zewde, who is a well-regarded diplomat representing Ethiopia at global, continental and regional organisations including the United Nations and the Intergovernmental Authority on Development, had also served as Ethiopia's ambassador to various countries. Throughout her career as a diplomat, Zewde held several positions representing the nation, reports Al Jazeera English.
Last week, parliament unanimously approved Abiy's nominations for his newly established 20-member Cabinet, half of whom are women, including the country's first female defense minister, Aisha Mohammed Musa.
She said the move was significant in numerous aspects.
According to local broadcaster FANA, the House of Peoples' Representatives and the House of the Federation would accept the resignation and appoint someone else in his place. "They're now going to be changed". Her resignation from the United Nations role this week is said to be to allow her take the new position of president. "Actually, it should have happened a long time ago, but the next best time is now".