He was he son of an Aberdeenshire laird, Johnston of Johnston and Caskieben, and on his mother's side a grandson of the seventh Lord Forbes. He is thought to have begun his university studies at one, or both, of the colleges at Aberdeen, but around 1608 he went to Italy and graduated M.D. at Padua in 1610. Afterwards he lived at Sedan, in the company of the exiled Andrew Melville, and in 1619 was in practice in Paris. He appears to have returned to England about the time of James I's death in 1625, and to have been in Aberdeen in about 1628. He met William Laud in Edinburgh at the time of Charles I's Scottish coronation (1633). In that year, he had published a volume entitled Cantici Salomonis paraphrasis poetica, which, dedicated to Charles I, brought him to Laud's notice. He was encouraged by Laud in his literary efforts, possibly as a strike against George Buchanan's reputation as a Latin poet. Johnston was appointed rector of King's College, Aberdeen, in June 1637. Four years later he died at Oxford, on his way to London at Laud's invitation.
Johnston left more than ten works, all in Latin. Only two of these, published in the same year, are notable: (a) his version of the Psalms (Psalmorum Davidis paraphrasis poetica et canticorum evangelicorum, Aberdeen, 1637), and (b) his anthology of contemporary Latin verse by Scottish poets (Deliciae poetarum Scotorum huius aevi illustrium, Amsterdam, 1637).
The full version of the Psalms was the result of Laud's encouragement. It was for some time a strong rival of Buchanan's work, though not superior to the latter. The Deliciae, in two small thick volumes of 699 and 575 pages, was a patriotic effort in imitation of the various volumes (under a similar title) which had been popular on the Continent during the second decade of the century. The volumes are dedicated by Johnston to John Scot of Scotstarvet, at whose expense the collected works were published after Johnston's death, at Middelburg (1642). Selections from his own poems occupy pages 439-647 of the first volume, divided into three sections, Parerga, Epigrammata and Musae Aulicae.
He published a volume of epigrams at Aberdeen in 1632. In these pieces he shows himself at his best. His sacred poems, which had appeared in the Opera (1642), were reprinted by Lauder in his Poetarum Scolorum musae sacrae (1839). The earliest lives are by Lauder and Benson (in Psalmi Davidici, 1741). Ruddiman's Vindication of Mr George Buchanan's Paraphrase (1745) began a pamphlet controversy regarding the merits of the rival poets.
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