McLellan was born in 1907 at Linmill, a fruit farm in Kirkfieldbank in the Clyde valley, the home of his maternal grandparents. He was educated at Bearsden Academy in Glasgow before studying philosophy at the University of Glasgow. However, he did not complete his degree because of ill-health. He met his future wife, Kathleen Heys while climbing in the Lake District. They were married in 1938 and settled in Arran where they lived modestly on his income as a playwright.

McLellan dedicated himself to writing in Scots, the living language of the communities he grew up in. As he himself said in an article written for The Scottish Field in 1956: “When I found that what I wanted most to do in life was to write for a Scottish Theatre I knew that I should always be poor, but I consoled myself with the thought that I could at least live where I liked.” Drama in Scotland was undergoing a resurgence in the early twentieth century and the Glasgow Repertory Company was one of the outlets for talented new Scottish writers such as McLellan. Respected amateur companies like the Scottish National Players and the Curtain Theatre also premiered his work. His first play was the one act comedy, Jeddart Justice, set in the feuding Border country of the sixteenth century. (Jeddart, or Jedburgh, Justice is condemnation without a hearing.) It was produced in 1933 in Glasgow by the Curtain Theatre. This was followed by Toom Byres (Empty Cowsheds) in 1936. His masterpiece, Jamie the Saxt was first performed at the Curtain Theatre in 1937, making Duncan Macrae, who played Jamie, one of Scotland’s most celebrated actors, an overnight star. It was revived by the Citizens Theatre, Glasgow in 1947 and by the Scottish Theatre Company on Scottish tour in 1982, and received its English premiere in 2007 at the Finborough Theatre.

He saw wartime service in the Royal Artillery from 1940—1946 and after the war, returned to exciting new possibilities for Scottish drama. Glasgow Unity Theatre was formed in 1941 and the Citizens Theatre in 1943. His work was enthusiastically embraced by both. Glasgow Unity Theatre in 1946 saw the first performance of Torwatletie. In The Flouers o Edinburgh (1948), he explicitly and hilariously explored the sociolinguistic tension between Scots and English in Scotland, but his satirical treatment was rooted in a deep love for the language of Lowland Scotland. His belief in Scots as medium for Scottish drama, and the vigour, lyricism and humour that he found in it, did much to extend the literary range of a language whose registers had been eroded since the Union of the Crowns. But in spite of his prolific output, he did not receive the acclaim he deserved because writing in Scots has in the past been perceived, however unjustly, as a barrier to widespread and frequent productions.

Few dramatists have matched Robert McLellan’s skill at deploying the vigorous vocabulary of Scotland and his language is the beautifully lyrical, witty and intelligent Scots of his time. As a younger Scots playwright, Donald Campbell, said: “Robert McLennan wasn’t just a playwright, he was something else — something different, something special. He was a superb lyric poet who happened to have the additional gift of a theatrical imagination”.

In addition to plays, McLellan also wrote five radio plays, The Linmill Stories, two poetic works — The Arran Burn and Sweet Largie Bay which was awarded The Scottish Arts Council’s Poetry Prize in 1956, and a history of the Isle of Arran published in 1969.

Robert McLellan played an active part in the community as a member of Arran District Council and as a president of the District Councils Association of Scotland. He also worked in the interests of his fellow writers as chair of the Scottish Sub-Committee of the League of Dramatists, chair of the Scottish Sub-Committee of the Society of Authors, Honorary Vice Preses (sic) of the Lallans Society and Honorary Vice President of the Scottish Society of Playwrights.

He was awarded the Order of the British Empire in 1978.

He died in 1985 and is buried on the Isle of Arran.