Montgomery was born in Perth, Western Australia, to a Canadian mother Judith Barrett-Lennard and a New Zealander father Scott Montgomery-Harvey. He has one younger sister, Saskia. His parents worked in the screen industry in Australia. Montgomery began performing on screen and in theater at the age of nine. Montgomery attended Mount Lawley Senior High School in his hometown. When Montgomery was in Year 12, his fellow students voted for him to be "The most likely student to become a Hollywood star" in the annual yearbook. Montgomery continued his studies in the dramatic arts throughout his secondary schooling. Montgomery completed his acting degree at the Western Australian Academy of Performing Arts at Edith Cowan University in 2015. In an Instagram post, Dacre said that he was a lost kid. He had a hard time in school, due to the fact he was suffering from anxiety at a young age. He claims he was bullied through his school years due to his weight. He also failed his drama exams in high school. At the age of 18, he was fired from his job, and was told to leave his drama school.
