He gained power on 20 April 2021, succeeding his father and predecessor, Idriss Déby, who died in action while commanding troops in the Northern Chad offensive.

[7] Mahamat Déby's third wife, Dahabaya Oumar Souni, is a journalist and media advisor who is considered the First Lady of Chad.

Upon his return he was enrolled in second promotion of semi direct of the school of officer inter arme and later was appointed to the service branch of the Security of State Institutions (DGSSIE), as a deputy commander of its infantry group.

[14] Some political actors within Chad labeled the installing of the transitional military government a coup, as the constitutional provisions regarding the filling of a presidential vacancy were not followed.

[16] France, one of Chad's main foreign policy allies, defended the development as necessary, citing the "exceptional circumstances" caused by the rebellion.

[19] Despite this recognition, FACT rebels have issued an open threat to the new government, stating that "Chad is not a monarchy" and vowing to continue fighting until they reach N'Djamena and depose Mahamat Déby from power.

[25] Without major opposition and civilian participation in the national dialogue, it concluded to extend the transition and postpone elections until at least October 2024.

[25][26] In March 2023, Déby pardoned 380 jailed FACT members, many of whom were among the 400+ sentenced to life in prison for the death of his father,[27] in an attempt to have the group join peace talks, following their non-participation in the national dialogue.

[35] Despite the 2021 termination of Operation Barkhane, France has not demonstrated any wavering in their support of Déby and continues to position troops in the region,[36][37] despite having moved the remaining core of their Sahel forces to Niger.

[41] Despite significant territorial losses, Boko Haram has adapted by shifting to asymmetric tactics such as ambushes and suicide bombings, continuing to target both military and civilian sites in the Lake Chad region.

[45] On 13 January 2024, Déby was announced as the nominee of the Patriotic Salvation Movement for the 2024 Chadian presidential election by party secretary Mahamat Zene Bada.

[46] After a raid on the home of his main opponent, Yaya Dillo Djérou, by government forces on 28 February 2021 in which five of his relatives were killed,[47] three other candidates had withdrawn in protest and fear of similar consequences.

[48] The attack was condemned by the African Union, while French Minister for Europe and Foreign Affairs Jean-Yves Le Drian called for an impartial investigation into the incident.

[55] In January 2024, a preliminary investigation was opened by the national financial prosecutor's office in France (PNF), for embezzlement of public funds and concealment “concerning the clothing expenses" of Déby in Paris.