Mirchoff was born in Seattle, Washington. Two days after his birth, the family moved to Victoria on Vancouver Island in Canada. His father, Bill, born in California, is an orthopedist, and his mother, Kelly, a Seattle native, is a homemaker. Mirchoff has an older brother, Luke (b. 1985), a lacrosse player, and a younger sister, Rianna (b. 1992). He is a graduate of Mount Douglas Secondary School. Mirchoff holds dual citizenship of the United States and Canada.

At the age of 20 he moved to Los Angeles. From 2011 to 2018 he was in a relationship with actress and dancer Janine Mason, winner of the fifth season of So You Think You Can Dance. In August 2021 Mirchoff got engaged to psychologist Jennie Mainen, but they have since ended the relationship.

Mirchoff's acting career began with the play Bubbly Stiltskin, and in 2006 he appeared in the film Scary Movie 4. In 2009 he played the lead role in the horror film The Grudge 3. That same year Mirchoff joined the cast of the series Desperate Housewives, playing the role of Danny Bolen, and in 2011 he got the lead role in the series Awkward and appeared in the film I Am Number Four. In 2013 Mirchoff starred in the TV film Wizards Return: Alex vs. Alex and in the independent film The Secret Lives of Dorks. In 2014 Mirchoff appeared in the lead role in the film Poker Night. In 2017 he played the role of Brad in the film Flatliners.