While Pope Francis is the pope who instituted the memorial of Mary, Mother of the Church, it was St. Paul VI who affirmed this title at the end of the Second Vatican Council.
This title can be traced back to the 4th century in the writings of St. Ambrose and can be found in various other writings over the years, especially in the 19th and early 20th centuries.
St. Paul VI made it a point to specifically mention this title at the end of the Second Vatican Council, explaining that bishops, priests and laity were asking him to affirm this ancient title.
St. Paul VI then made an official proclamation of this title, affirming its place in the life of the Church.
He explains how this title affirms that she who is Mother of God, is also our Mother.
Mary is our heavenly mother and this title emphasizes this connection, assuring us that we were are not orphans, but sons and daughters of God and the Virgin Mary.