Born on April 5, 1975, Rebecca Hornbrook grew up in Barrie, Ontario. While attending Innisdale Secondary School, Hornbrook excelled at science and math which encouraged her to study science after graduation. She attended York University in Toronto where she earned her undergraduate degree in chemistry while studying to become a high school science teacher. Towards the end of her undergraduate career, Hornbrook, who spent summers working at an atmospheric chemistry lab affiliated with York University's Centre For Atmospheric Chemistry, decided to pursue her graduate degree.
After graduating with her Bachelor of Education in Chemistry and Mathematics as well as her Bachelor of Science in Chemistry with honors in atmospheric chemistry, Hornbrook began her Doctor of Philosophy in Chemistry at York University. Throughout her graduate education, she worked alongside Jochen Rudolph conducting research on Volatile organic compounds and their role in the chemistry of the troposphere.  Hornbrook published a total of seven papers before graduating with her Ph.D in chemistry from York University in 2005.
Hornbrook is the recipient of two Governor General's Academic Medals which are awarded to the student who graduates a Canadian school with the highest grades. She was awarded the Gold Medal in 2006 for her Ph.D dissertation completed the year before at York University and the Bronze Medal in 1994 while attending Innisdale Secondary School.