After he had spent several years in the role of senior assistant, he held a series of senior positions as president of the Pontifical Council for Justice and Peace, president of the Pontifical Council Cor Unum, and prefect of the Sacred Congregation for Bishops Pope Paul VI made him a cardinal in 1977, Pope John Paul II promoted him to the rank of cardinal bishop in 1986, and his peers elected him dean, the highest office in the College of Cardinals, in 1993.

[2] He entered the minor seminary in Ouidah at age fourteen and was ordained to the priesthood on 14 January 1951 in Lomé, Togo, by Archbishop Louis Parisot of Cotonou.

[6] As archbishop, he attended all four sessions of the Second Vatican Council (1962–1965), where he first became friends with the future Pope John Paul II.

He was made a member of the order of cardinal deacons and assigned the deaconry of Sacro Cuore di Cristo Re.

[8] On 4 September 1978, he was named President of the Pontifical Council Cor Unum by Pope John Paul I, the only administrative appointment of his month-long papacy.

[7] On the pope's behalf he managed appointments of conservative prelates in dioceses that did not welcome them in the Netherlands and Switzerland, removed an outspoken liberal French bishop, contended with Latin American advocates for the rights of indigenous peoples, and the excommunication of Marcel Lefebvre, with whom he had worked in Africa in the 1960s.

[16] He ended his service as prefect of the Congregation for Bishops and President of the Pontifical Commission for Latin America on 25 June 1998.

Less than a year later, in April 1999, he endorsed a views of Cardinal Vincenzo Fagiolo that bishops need to consider themselves married to their sees and expect their relationship to be lifelong.

He warned against allowing African priests to relocate permanently in Europe, to "roam the dioceses of the Western world more in search of their own material comfort than out of genuine pastoral zeal".

"[23] Gantin died at Pompidou Hospital in Paris after a long illness on 13 May 2008, less than a week after being transferred there from Benin and five days after his 86th birthday.

[25] In May 2013, Vatican officials inaugurated a chair named for him devoted to "Socializing Policy in Africa" at the Pontifical Lateran University.

[25][27] On 30 January 2025, the Episcopal Conference of the Italian Region of Lazio, which includes the Diocese of Rome, has issued a favourable opinion for the opening of the cause for beatification of Gantin, the first African to lead the Dicastery for Bishops in the Vatican.