For the second successive Games, MacLennan upstaged Chinese favourites Li Dan and 2008 Olympic champion He Wanna to earn Canada their second gold of the Rio Games.
“It's so surreal,” she said afterwards. “I'm so excited and really proud. I'm so grateful to have had the opportunity to come out here to compete at the Olympic again. I can't even describe it."
MacLennan trailed Li during qualifying, but she soared to the top of the podium with a gravity-defying routine in the final that earned her a score of 56.465 and wild cheers from the crowd.
Li followed her on to the apparatus and had been expected to leapfrog the Canadian in the standings. However, after registering a score of 55.885, she was not only beaten to first place but pipped to the silver medal by Great Britain's Bryony Page.
Page, who finished seventh in qualifying, was in floods of tears upon realising that she had earned Britain their first ever medal in trampolining.
Once the last competitor, surprise qualifying leader Tatsiana Piatrenia (BLR), had finished her display, Page broke down again as the result confirmed she had won silver with a score of 56.040.
He's hopes of becoming the first trampoline gymnast to win two Olympic golds ended in disappointment as she missed out on bronze by 0.315 of a point.
“I think I performed well enough to medal,” said He. “I feel very proud of myself, but at the same time, I feel like it's a great pity that I didn't medal. In China, the competition is so fierce. My coach told me that even being here, at these Games, is a huge success. I should be proud of myself.”