Rosie MacLennan trains at Skyrider's Trampoline Place in Richmond Hill, Ontario with coach, David Ross. She has competed internationally at various levels since 1999. In 2006 she paired with her training partner, the double Olympian Karen Cockburn, in synchronized trampoline and since then the pair have dominated the event internationally winning eight consecutive World Cup events including the World Cup Finals in Birmingham in 2006. In the 2007 World Championships in Quebec City they again won the event. The pair hold the current female synchronized trampoline routine world record for difficulty with a DD of 14.20 which they scored in April 2007 at the Lake Placid Trampoline World Cup. Her results at the 2007 World Championships qualified her for the 2008 Summer Olympics in Beijing.
Following the 2007 World Championships, MacLennan came in second place in the Good Luck Beijing International Invitational Tournament, a competition held to test the facilities and organization for the 2008 Beijing Olympic Games. 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. In the trampoline preliminary competition, she qualified in 3rd place for the Finals but eventually finished in 7th place.
After the 2008 Olympics, MacLennan won the 2009 Canadian Women's Individual title. She came in 4th place for individual trampoline in the 2009 Trampoline World Championships in St Petersburg and 3rd place for individual trampoline in the 2010 Trampoline World Championships in Metz. In 2011, she again won the Canadian Championships and came in 1st place at the Pan American Games in Guadalajara, Mexico and 2nd place in the 2011 Trampoline World Championships in Birmingham which won a place for Canadian women in the Trampoline event for the 2012 London Olympics.
Her next major competition was at the 2012 Gymnastics Olympic Test Event held in the same location as the Olympics. MacLennan won that event against some of the Olympic competitors that she would later face. In May 2012 she suffered a concussion and had to be cautious in her training missing the 2012 Canadian Trampoline Championships. 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. This was the first and only gold medal for Canada at these games and the first Canadian trampoline gold medal ever.