MacMillan was born at Kilwinning, in North Ayrshire, but lived in the East Ayrshire town of Cumnock until 1977.
He studied composition at the University of Edinburgh with Rita McAllister, and at Durham University with John Casken, where he gained a PhD degree in 1987. He was a lecturer in music at the Victoria University of Manchester from 1986 to 1988. After his studies, MacMillan returned to Scotland, composing prolifically, and becoming Associate Composer with the Scottish Chamber Orchestra, often working on education projects.
He came to the attention of the classical establishment with the BBC Scottish Symphony Orchestra's premiere of The Confession of Isobel Gowdie at the Proms in 1990. Isobel Gowdie was one of many women executed for witchcraft in 17th century Scotland. According to the composer, "the work craves absolution and offers Isobel Gowdie the mercy and humanity that was denied her in the last days of her life".
The work's international acclaim spurred more high-profile commissions, including a percussion concerto for fellow Scot Evelyn Glennie, Veni, Veni, Emmanuel. It was premiered in 1992 and has become MacMillan's most performed work. He was also asked by Mstislav Rostropovich to compose a cello concerto, which was premiered by Rostropovich himself in 1997.
Further successes have included his second opera The Sacrifice, commissioned by Welsh National Opera, Autumn 2007, which won a Royal Philharmonic Society Award, and the St John Passion jointly commissioned by the London Symphony Orchestra and Boston Symphony Orchestra and conducted by Sir Colin Davis at its world premiere in April 2008. He was awarded the British Composer Award for Liturgical Music, for his Strathclyde Motets, in December 2008.
MacMillan's music is infused with the spiritual and the political. His Roman Catholic faith has inspired many of his sacred works; for example, Magnificat (1999), and several Masses. This central strand of his life and compositions was marked by the BBC Symphony Orchestra in early 2005, with a survey of his music entitled From Darkness into Light. MacMillan and his wife are lay Dominicans, and he has collaborated with Michael Symmons Roberts, a Catholic poet, and also Rowan Williams, the Archbishop of Canterbury. Perhaps his most political work is Cantos Sagrados (1990), a setting of Latin American poetry by Ariel Dorfman and Ana Maria Mendoza, combining elements of liberation theology with more conventional religious texts. MacMillan has explicitly stated that his aim in writing this work was to emphasise 'a deeper solidarity with the poor of that subcontinent' in the context of political repression.
Scottish traditional music has also had a profound musical influence, and is frequently discernible in his works. When the Scottish Parliament was reconvened in 1999 after 292 years, a fanfare composed by MacMillan accompanied the Queen into the chamber. Weeks after the opening ceremony, MacMillan launched a vigorous attack on sectarianism in Scotland, particularly anti-Catholicism, in a speech entitled "Scotland's Shame".
His Mass of 2000 was commissioned by Westminster Cathedral and contains sections which the congregation may join in singing. Similarly, the St Anne's Mass and Galloway Mass do not require advanced musicianship, being designed to be taught to a congregation.
One of his most important commissions (by the Bishops' Conferences of England & Wales and of Scotland) was to write a new mass setting for choir and congregation to be sung at two of the three masses celebrated by Pope Benedict XVI during his Apostolic and state visit to Great Britain in 2010. First sung at mass at Bellahouston Park, Glasgow, on 16 September it was sung again at the mass and beatification of John Henry Newman at Cofton Park, Birmingham, on 19 September). He was also commissioned to write a setting of the text Tu es Petrus (Matthew 16:18) for the Pope's entry at mass at Westminster Cathedral on 18 September.
MacMillan was composer and conductor with the BBC Philharmonic from 2000 to 2009, following which he took up a position as principal guest conductor with the Netherlands Radio Chamber Philharmonic. His collaboration with Michael Symmons Roberts continued with his second opera, The Sacrifice (based on the ancient Welsh tales of the Mabinogion), being premiered by Welsh National Opera in Autumn 2007. Sundogs, a large-scale work for a cappella choir, also using text by Symmons Roberts, was premiered by the Indiana University Contemporary Vocal Ensemble in August 2006.
He is an Honorary Fellow of Blackfriars Hall, University of Oxford. He is patron of St Mary's Music School in Edinburgh, of the London Oratory School Schola Cantorum along with Simon Callow and HRH Princess Michael of Kent, and he has recently been appointed patron of The British Art Music Series along with Libby Purves and John Wilson, and of the Schola Cantorum of the Cardinal Vaughan Memorial School
He was appointed a Commander of the Order of the British Empire (CBE) in 2004, and a Knight Bachelor in 2015.
In 2008, he became Honorary Patron of London Chamber Orchestra's LCO New: Explore project, which explores links between music and other art forms and fosters emerging creative talent in composition. He also serves as the Honorary President of the Bearsden Choir.
MacMillan is married with two daughters and a son.After the Tryst (violin + piano - 1988)
Cantos Sagrados (choir + organ - 1989)
The Confession of Isobel Gowdie (orchestra - 1990)
The Berserking (piano concerto - 1990)
Veni, Veni, Emmanuel (percussion concerto - 1992)
Seven Last Words from the Cross (cantata: choir and strings - 1993)
Inés de Castro (opera, libretto: Jo Clifford – 1991–95)
Britannia! (orchestra - 1994)
Christus Vincit (1994), for SSAATTBB + Soli
Cello Concerto (1996)
The World's Ransoming (cor anglais and orchestra - 1996)
Symphony: Vigil (1997)
Quickening (soloists, chorus, orchestra - 1998)
Symphony No. 2 (1999)
Mass (choir, organ - 2000)
Cello Sonata No. 2, dedicated to Julian Lloyd Webber
The Birds of Rhiannon (orchestra + optional chorus, text: Michael Symmons Roberts - 2001)
O Bone Jesu (2001), for SSAATTBB + Soli
Symphony No. 3 "Silence" (2002)
Piano Concerto No. 2 (2003)
A Scotch Bestiary (organ and orchestra - 2004)
The Sacrifice (2007)
St John Passion (2008)
Piano Concerto No. 3 "The Mysteries of Light" (2008)
Violin Concerto (2009)
Oboe Concerto (2010)
Woman of the Apocalypse (2012)
St Luke Passion (2013)
Viola Concerto (2013)
Percussion Concerto No. 2 (2014)
Symphony No. 4 (2015)
European Requiem (2017)