Johnston was born in Dublin, the son of the physicist and political activist Roy H. W. Johnston, and studied for both a degree in music and a master's degree in Music and Media Technology at Trinity College Dublin (MusB 1982).
Initially, he had also studied at the Royal Irish Academy of Music, Dublin (flute with Doris Keogh, clarinet with Tim Hanafin, and composition with James Wilson).
In 1985 he was invited by the European Community Youth Orchestra to write an orchestral piece under the guidance of György Ligeti.
His use of the system is unusual among Irish composers, his compositional process typically includes a significant element of pre-planning.
[2] Stage Orchestral works Chamber music Solo instrumental Vocal Choral Electro-acoustic