On 30 August 2020, he was backed by the Future Movement of Lebanon and a group of former Lebanese prime ministers to take up the post of PM. He was nominated for the position while serving as the Lebanese ambassador to Germany in order to replace Hassan Diab who resigned on 10 August 2020 due to the outcry and uproar following the 2020 Beirut explosion which killed nearly 200 people. Mustapha was reported to have received support of major parties such as Free Patriotic Movement to form a new cabinet. Former PM Najib Mikati became the first politician to nominate Mustapha Adib for the position. He was also the only candidate to have been nominated for the position as of 30 August 2020 and his nomination was reported to have been approved by French President Emmanuel Macron who was due to arrive in Lebanon for peace talks and discussions about political reforms in Lebanon. On 31 August 2020, he was formally appointed as the new designated PM of Lebanon a day prior to the deadline (1 September 2020) set by the French President who demanded for a political change in the country free of corruption. His appointment came in just few hours prior to the visit by Emmanuel Macron. He received support from 90 of 120 MPs to become the new Prime Minister of Lebanon and to form the government on 31 August 2020.