She has also won five silver and four bronze medals in World Championship competition in both the individual and synchro events.

[3] Her grandfather was selected as a gymnast for the 1940 Summer Olympics in Tokyo, but was unable to compete as the games were cancelled due to the outbreak of World War II.

In June 2008, she was selected to join Karen Cockburn and Jason Burnett as one of Canada's three trampoline gymnasts at the 2008 Summer Olympics in Beijing.

[12] However at the 2012 Summer Olympics in London, she put on her best performance ever with a finals routine of 57.305, which was the gold-medal winning score for Canada.

China's He Wenna fell at the end of her performance but narrowly beat out MacLennan's Canadian teammate Karen Cockburn for the bronze medal.

[17] During training before the 2015 Pan American Games, she sustained a mild concussion when she landed on the side of a trampoline.

[18] She won the women's trampoline event at the Pan American Games two weeks later, then undertook physical and cognitive rest to recover from the concussion.

She stated that she was "having some issues with spatial awareness" after the concussion with symptoms including headaches, dizziness, and photosensitivity.

[20] She conducted exercises with a vestibular ocular therapist to restore her balance and timing, and began doing flips again by November 2015.