His most recent solo exhibition was ‘Future Present’ at The Ulster University Gallery, Belfast (2019). He has exhibited in numerous group and curated exhibitions across Ireland and UK, most recently ‘New Exits’ at The MAC Belfast (2022-23). Millen was selected for the last two BEEP Painting Biennials, Elysium Gallery, Swansea (2020 & 2022). He has received several arts awards including Freelands Foundation Funding (2020), the Hennessy Craig Scholarship, Royal Hibernian Academy, Dublin (2009) and numerous Arts Council of Northern Ireland Awards. His work is also included in several public art collections including the National Self-Portrait Collection of Ireland, Limerick University and the Arts Council NI.