In 1560 or 1561 he returned to Scotland, and in April 1562 we find him installed as tutor to the young queen Mary, who was accustomed to read Livy with him daily. Buchanan now openly joined the Protestant, or Reformed Church, and in 1566 was appointed by the earl of Murray principal of St Leonard's College, St Andrews. Two years before he had received from the queen the valuable gift of the revenues of Crossraguel Abbey. He was thus in good circumstances, and his fame was steadily increasing. So great, indeed, was his reputation for learning and administrative capacity that, though a layman, he was made moderator of the general assembly in 1567. He had sat in the assemblies from 1563.

Buchanan accompanied the regent Murray into England, and his Detectio (published in 1572) was produced to the commissioners at Westminster. In 1570, after the assassination of Murray, he was appointed one of the preceptors of the young king, and it was through his tuition that James VI. acquired his scholarship. While discharging the functions of royal tutor he also held other important offices. He was for a short time director of chancery, and then became lord privy seal, a post which entitled him to a seat in the parliament. He appears to have continued in this office for some years, at least till 1579. He died on the 28th of September 1582.

His last years had been occupied with two of his most important works. The first was the treatise De Jure Regni apud Scotos, published in 1579. In this famous work, composed in the form of a dialogue, and evidently intended to instil sound political principles into the mind of his pupil, Buchanan lays down the doctrine that the source of all political power is the people, that the king is bound by those conditions under which the supreme power was first committed to his hands, and that it is lawful to resist, even to punish, tyrants. The importance of the work is proved by the persistent efforts of the legislature to suppress it during the century following its publication. It was condemned by act of parliament in 1584, and again in 1664; and in 1683 it was burned by the university of Oxford. The second of his larger works is the history of Scotland, Rerum Scoticarum Historia, completed shortly before his death (1579), and published in 1582. It is of great value for the period personally known to the author, which occupies the greater portion of the book.

The earlier part is based, to a considerable extent, on the legendary history of Boece. Buchanan's purpose was to "purge" the national history "of sum Inglis lyis and Scottis vanite" (Letter to Randolph), but he exaggerated his freedom from partisanship and unconsciously criticized his work when he said that it would "content few and displease many."

Buchanan is one of Scotland's greatest scholars. For mastery over the Latin language he has seldom been surpassed by any modern writer. His style is not rigidly modelled upon that of any classical author, but has a certain freshness and elasticity of its own. He wrote Latin as if it had been his mother tongue. But in addition to this perfect command over the language, Buchanan had a rich vein of poetical feeling, and much originality of thought. His translations of the Psalms and of the Greek plays are more than mere versions; the smaller satirical poems abound in wit and in happy phrase; his two tragedies, Baptistes and Jephthes, have enjoyed from the first an undiminished European reputation for academic excellence. In addition to the works already named, Buchanan wrote in prose Chamaeleon, a satire in the vernacular against Maitland of Lethington, first printed in 1711; a Latin translation of Linacre's Grammar (Paris, 1533); Libettus de Prosodia (Edinburgh, 1640); and Vita ab ipso scripta biennio ante mortem (1608), edited by R. Sibbald (1702). His other poems are Fratres Fraterrimi, Elegiae, Silvae, two sets of verses entitled Hendecasyllabon Liber and Iambon Liber; three books of Epigrammata; a book of miscellaneous verse; De Sphaera (in five books), suggested by the poem of Joannes de Sacrobosco, and intended as a defence of the Ptolemaic theory against the new Copernican view.

There are two editions of Buchanan's works: - (a) Georgii Buchanani Scoti, Poetarum sui seculi facile principis, Opera Omnia, in two vols. fol., edited by Ruddiman (Edinburgh, Freebairn, 1715); (b) edited by Burman, 4to, 1725. The Vernacular Writings, consisting of the Chamaeleon (u.s.), a tract on the Reformation of St Andrews University, Ane Admonitioun to the Trew Lordis, and two letters, were edited for the Scottish Text Society by P. Hume Brown. The principal biographies are: - David Irving, Memoirs of the Life and Writings of George Buchanan (Edinburgh,1807 and 1817); P. Hume Brown, George Buchanan, Humanist and Reformer (Edinburgh, 1890), George Buchanan and his Times (Edinburgh, 1906); Rev. D. Macmillan, George Buchanan, a Biography (Edinburgh, 1906). Buchanan's quatercentenary was celebrated at different centres in Scotland in 1906, and was the occasion of several encomia and studies. The most important of these are: George Buchanan: Glasgow Quatercentenary Studies (Glasgow, 1906), and George Buchanan, a Memoir, edited by D.A. Millar (St Andrews, 1907). A verse translation of the Baptistes, entitled Tyrannicall-Government Anatomized (1642), has been attributed to Milton; its authorship is discussed in the Glasgow Quatercentenary Studies. The records of Buchanan's trial, discovered by the Portuguese historian, G.J.C. Henriques, were published by him under the title George Buchanan in the Lisbon Inquisition. The Records of his Trial, with a Translation thereof into English, Facsimiles of some of the Papers, and an Introduction (Lisbon, 1906).