Mortimer competed in the 2008 Canadian Olympic Trials as an able-bodied swimmer, and retrained as a paraswimmer after a trampolining accident.

She began competing at the international level in 2010 with an SM10 classification, setting world records in the process (one of which she broke again in qualifying for the 2012 Paralympics).

[7] A serious injury at a competitive trampolining event shattered most of the bones in Mortimer's feet;[2][8] she was nationally ranked in the sport.

[10] Mortimer wears an elastic band which she snaps when aware of a negative thought; "incredibly hard on myself", it helps her realize the frequency of her "down" thinking and changes her mental perspective.

It's way harder";[10] Mortimer said that she knew "truckloads of able-bodied athletes who constantly make fun of the Paralympics".

[12][note 1] Between the Trials and the Paralympics, she won the 100-metre backstroke in July at the Canada Cup;[13] this result surprised Mortimer, who had a "rough time" with her feet the previous month.

[16] On August 30, the first day of the competition, Sophie Pascoe of New Zealand won the SM10 200-metre individual medley; Mortimer took the silver medal.

[17] For a half-hour before the competition, Mortimer was "bawling her eyes out";[17] she told The Canadian Press, "[Being] here and meeting all these people made me realize I am so incredibly grateful for the experience I’ve had as a result of my accident.

[18] She won the gold medal in the final on September 4, setting a world record of 1:05.90 after overcoming Pascoe's half-second lead at the turn.

[21][22] With Katarina Roxon, Morgan Bird and Brianna Nelson in the medley relay, Mortimer's team finished seventh and last.

[24] Following Mortimer's last individual event at London she said that although she intended to try for the 2013 World Aquatics Championships, the 2015 Parapan American Games, and the 2016 Paralympics,[16] she takes life year-by-year.

[9] Mortimer said that Paralympic athletes received no prize money, despite thousands of dollars given by the government to winning Olympians and their coaches.

[9] Mortimer's return to the pool included the April 9, 2014 Cam Am Para Swimming competition, where she won three gold medals.

[35] Although day-to-day funding in the Netherlands is similar to that in Canada, the former offers prize money; in the 2012 Paralympics, Mortimer would have won $60,000 for her gold medals alone.

"[37] The Big Splash (from Swimming Canada) Ontario Sport Award (from Sport Alliance Ontario) Queen Elizabeth II Diamond Jubilee Medal (2012)[43][note 3] Mortimer, Melissa Tancredi and Cory Conacher were nominated for the 2013 Golden Horseshoe Athlete of the Year for residents of Hamilton or Burlington.

[45] Columnist Dave Feschuk of the Toronto Star listed Mortimer as a contender for the 2012 Lou Marsh Trophy (won by Christine Sinclair).

[46][47] In 2012, Mortimer illustrated The Night I Met the Boogie Man, written by her twin sister Julia,[8][48] and created murals.

[55] She returned to co-host in-studio segments with Scott Russell for the opening and closing ceremonies and daily highlights programs, during the 2022 Winter Paralympics.

In November 2008, shortly after the Olympic trampoline trials, 15-year-old Mortimer missed the sponge pit after bouncing on a mini-trampoline and fell 15 feet (4.6 m) onto a concrete pad;[3] her parents, spectators at the competition, saw the accident.

After the accident her weekly threshold at the local YMCA pool was 30 to 40 kilometres (19 to 25 mi) and four sessions in the weight room.

"[8] Sportsnet Magazine chose Mortimer for its 2013 "Beauty of Sport" issue, and she was photographed in Las Vegas wearing a bikini.