[3] He was raised in a strict Presbyterian family on the island of Raasay, immersed in Gaelic culture and literature from birth, but abandoned religion for socialism.

His later poem Hallaig, published 1954, achieved "cult status"[4]: 134  outside Gaelic-speaking circles for its supernatural representation of a village depopulated in the Highland Clearances and came to represent all Scottish Gaelic poetry in the English-speaking imagination.

[9]: 10  His brothers were John (1910–1970), a schoolteacher and later rector of Oban High School, who was also a piper;[7][5]: 27  Calum (1915–1960), a noted folklorist and ethnographer; and Alasdair (1918–1999) and Norman (c.1917–c.1980), who became general practitioners.

[13]: 231–2  Both his mother's and father's families contained individuals who were considered accomplished by their communities, whether through formal education or extensive knowledge of the oral tradition.

[8]: 17 [18]: 8 MacLean later said that he had abandoned religion for socialism at the age of twelve,[8]: 17  as he refused to accept that a majority of human beings were consigned to eternal damnation.

[3] John MacInnes has argued that his evangelical Presbyterian background was an important influence on his choice of Gaelic as the medium for his poetry and the manner of its expression.

[3][12] After the Spanish Civil War broke out in 1936, he considered volunteering to fight in the International Brigades; according to his daughter, he would have gone if not for the poverty of his family and his own responsibilities as their provider.

MacLean cultivated friendships with Scottish Renaissance poets, including MacDiarmid, Robert Garioch, Norman MacCaig, Douglas Young, and George Campbell Hay.

[8][18]: 5 [iv] MacLean wrote a few poems about the war in which he challenged the traditional Gaelic exaltation of heroism, exemplified by the lament for Alasdair of Glengarry; he viewed physical courage as morally neutral, since it was also valued by Nazis and used for evil ends.

[8]: 32 [25]: 4  During this period, he frequently reviewed poetry and continued to make friends in literary circles, including the younger poets Iain Crichton Smith and George Mackay Brown.

[30] In 1956, MacLean was offered the position of head teacher of Plockton High School in Wester Ross, not far from where his paternal grandmother's family had lived.

"[31][vii] MacLean set high academic expectations for his students[21] and also promoted shinty; in 1965, the Plockton team won the cup for Ross and Cromarty.

[21] During his later years, he published few poems due to his "concern with quality and authenticity over quantity";[33][31] his family responsibilities and career left him little spare time to write.

[43]: 4 [ix] For 1,500 years, Scottish Gaelic literature had developed a rich corpus of song and poetry across "literary, sub-literary, and non-literate" registers; it retained the ability to convey "an astonishingly wide range of human experience".

[41]: 67  Of all poetry, MacLean held in highest regard the Scottish Gaelic songs composed before the nineteenth century by anonymous, illiterate poets and passed down via the oral tradition.

[8]: 17 MacLean once said that various Communist figures meant more to him than any poet, writing to Douglas Young in 1941 that "Lenin, Stalin and Dimitroff now mean more to me than Prometheus and Shelley did in my teens".

[25]: 3  After reading A Drunk Man Looks at the Thistle by MacDiarmid, MacLean decided to try his hand at extended narrative poetry, resulting in the unfinished An Cuilthionn.

[3] In his poetry, MacLean emphasized the struggle between love and duty, which was personified in the poet's difficulty in choosing between his infatuation with a female figure, Eimhir, and what he sees as his moral obligation to volunteer in the Spanish Civil War.

[xiv] Ronald Black suggested that "duty [is not]... a comprehensible emotion nowadays" and therefore "the greatest universal in MacLean's verse is the depiction of that extraordinary psychosis which is called being in love".

[2] According to John MacInnes, MacLean put the much-denigrated Gaelic language and tradition in its proper place, which has a profound effect on Gaelic-speaking readers and is fundamental to their reading of his poetry.

[2][33] MacLean frequently compared the injustice of the Highland Clearances with modern-day issues;[4]: 133  in his opinion, the greed of the wealthy and powerful was responsible for many tragedies and social problems.

[36]: 2 [9]: 9  To English-speakers, MacLean remained virtually unknown until 1970, when issue 34 of Lines Review was dedicated to his work and some of his poems were reproduced in the anthology Four Points of the Saltire.

[41]: 172  In 1989, a further compilation of his poetry, O Choille gu Bearradh / From Wood to Ridge: Collected Poems in Gaelic and English won him lasting critical acclaim.

[41]: 72  His readings were described as deeply moving even by listeners who did not speak Gaelic;[8]: 17  according to Seamus Heaney, "MacLean's voice had a certain bardic weirdness that sounded both stricken and enraptured".

[45] Beginning with the famous line, "Time, the deer, is in the wood of Hallaig",[xviii] the poem imagines the village as it was before the Clearances, with the long-dead eternally walking through the trees.

Translated and promoted by Irish Nobel Prize Laureate Seamus Heaney,[23]: 13  Hallaig achieved "cult status"[4]: 134  and came to symbolize Scottish Gaelic poetry in the English-speaking imagination.

[40]: 417  According to Iain Crichton Smith, translator of MacLean's poetry, Dàin do Eimhir was "the greatest Gaelic book of this century", an assessment with which Christopher Whyte agreed.

[41]: 67–68  MacInnes concedes that MacLean does not cater to his readers; however, in his opinion it would be incorrect to call the poetry elitist because of its "artistic sincerity", speaking "with affective directness and a simple passionate intensity".

[25]: 5 A film, Hallaig, was made in 1984 by Timothy Neat, including a discussion by MacLean of the dominant influences on his poetry, with commentary by Smith and Heaney, and substantial passages from the poem and other work, along with extracts of Gaelic song.

[6]: 193  The poem also forms part of the lyrics of Peter Maxwell Davies' opera The Jacobite Rising;[60] and MacLean's own reading of it in English and in Gaelic was sampled by Martyn Bennett in his album Bothy Culture for a track of the same name.