From here: ewtn.com/library/ANSWERS/DOXOLOG.HTM
It was a prayer from the Mass, not originally in the Bible.
" In the early Church, the Christians living in the eastern half of the Roman Empire added the doxology “for thine…” to the Gospel text of the Our Father when reciting the prayer at Mass. Evidence of this practice is also found in the “Didache” (Teaching of the Twelve Apostles), a first century manual of morals, worship and doctrine of the Church. Also, when copying the Scriptures, Greek scribes sometimes appended the doxology onto the original Gospel text of the Our Father, however, most texts today would omit this inclusion, relegate it to a footnote, or note that it was a later addition to the Gospel."
" Interestingly, the English wording of the Our Father that we use today reflects the version mandated for use by Henry VIII (while still in communion with the Catholic Church), which was based on the English version of the Bible produced by Tyndale (1525)."
" Official “Catholic” Bibles including the Vulgate, the Douay-Rheims, the Confraternity Edition, and the New American have never included this doxology."
" The irony of this answer is that some Protestants sometimes accuse Catholics of not being “literally” faithful to Sacred Scripture and depending too much on tradition. In this case, we see that the Catholic Church has been faithful to the Gospel text of the Our Father, while Protestant Churches have added something of tradition to the word of Jesus."