The beginning of the concept that she is a Queen is found in the annunciation narrative. For the angel tells her that her Son will be King over the house of Jacob forever. So she, His Mother, would be a Queen.
The Fathers of the Church soon picked up these implications. A text probably coming from Origen (died c. 254: cf. Marian Studies 4, 1953, 87) gives her the title domina, the feminine form of Latin dominus. That same title also appears in many other early writers, e.g. , St. Ephrem, St. Jerome, St. Peter Chrysologus. (cf. Marian Studies 4. 87-91. The word Queen appears abut the sixth century, and is common thereafter (Marian Studies, 4, 91-94. )
The titles of king or queen are often used loosely, for those beings that excel in some way. Thus we call the lion the king of beasts, the rose the queen of flowers. Surely Our Lady deserves the title richly for such reasons. But there is much more.
Some inadequate reasons have been suggested: She is the daughter of David. But not every child of a king becomes a king or queen. Others have pointed out that she was free from original sin. Then, since Adam and Eve had a dominion over all things (Genesis 1. 26) she should have similar dominion. But the problem is that the royalty of Adam and Eve was largely metaphorical.
The solidly theological reasons for her title of Queen are expressed splendidly by Pius XII, in his Radio message to Fatima, Bendito seja (AAS 38. 266): "He, the Son of God, reflects on His heavenly Mother the glory, the majesty and the dominion of His kingship, for, having been associated to the King of Martyrs in the unspeakable work of human Redemption as Mother and cooperator, she remains forever associated to Him, with a practically unlimited power, in the distribution of the graces which flow from the Redemption. Jesus is King throughout all eternity by nature and by right of conquest: through Him, with Him, and subordinate to Him, Mary is Queen by grace, by divine relationship, by right of conquest, and by singular choice [of the Father]. And her kingdom is as vast as that of her Son and God, since nothing is excluded from her dominion."
We notice that there are two titles for the kingship of Christ: divine nature, and "right of conquest", i.e. , the Redemption. She is Queen "through Him, with Him, and subordinate to Him." The qualifications are obvious, and need no explanation. Her Queenship is basically a sharing in the royalty of her Son. We do not think of two powers, one infinite, the other finite. No, she and her Son are inseparable, and operate as a unit.
Of the four titles Pius XII gave for her Queenship , we notice that two are closely parallel to those of Jesus: (1) He is king by nature, as God, she is Queen by "divine relationship" that is, by being the Mother of God. In fact her relation to her Son is greater than that of ordinary Mothers of Kings. For she is the Mother of Him who is King by very nature, from all eternity, and the relationship is exclusive, for He had no human father. Still further, the ordinary queen-mother gives birth to a child who later will become king. The son of Mary is, as we said, eternally king, by His very nature. He is king by right of conquest. (2) She too is Queen by right of conquest. We already saw that this title for Him means that He redeemed us from the captivity of Satan. She shared in the struggle and victory. Since the Pope expressed her dependence on Him in a threefold way—something we would have known anyway—then it is clear that he did not have in mind any other restriction which he did not express. So, with subordination, "by right of conquest" means the same for her as it does for Him.
The other two titles: (3)She is Queen by grace. She is full of grace, the highest in the category of grace besides her Son. (4)She is Queen by singular choice of the Father. A mere human can become King or Queen by choice of the People. How much greater a title is the choice of the Father Himself!
Pius XII added that "nothing is excluded from her dominion." As Mediatrix of all graces, who shared in earning all graces, she is, as Benedict XV said in a text already cited, "Suppliant omnipotence": she can obtain by her intercession anything that the all-powerful God can do by His own inherent power.
In the OT, under some Davidic kings, the gebirah, the "Great Lady", usually the Mother of the King, held great power as advocate with the king. Cf. 1 KGB 2:20, where Solomon said to his Mother Bathsheba, seated on a throne at his right: "Make your request, Mother, for I will not refuse you." Here is a sort of type of Our Lady.