McDonough is the youngest son of four boys, born to Kathy, an Irish mother. McDonough spent much of his childhood living in Singapore, where his parents were working, where he was educated at Tanglin Trust School and the United World College of South East Asia. In his mid-teens, he left Singapore and moved back to Australia with his family. They settled in the Melbourne suburb of Plenty. McDonough finished his schooling at Eltham College of Education in Research. Blair and his brothers Wade, Gavin and Ryan were members of the band Snubrocket (later renamed Fifth Avenue), which was one of 30 that appeared in the music video clip for TISM's song "Thunderbirds Are Coming Out" in 1998. In 1999, Blair attended the National Youth Science Forum in Canberra.