McMorris competed at his first FIS Snowboard World Cup during the 2009–2010 season placing eighth in the big air event in Quebec City. He followed up with a World Cup victory in slopestyle in Calgary.
At the Winter X Games XV in Aspen, Colorado, he won a silver medal in the slopestyle competition finishing behind fellow Canadian Sebastien Toutant. He also won the 2011 World Snowboard Tour's Air & Style competition beating out Peetu Piiroinen, Werner Stock, and Nils Arvidsson in the final. McMorris is the first person in X Games history to score a 98.00 in snowboard slopestyle. McMorris achieved the score while defending his title in his final run at the 2013 Winter X Games. The medal had completed a double podium performance at the games where McMorris had also won the silver with his performance in the big air competition. This all occurred just several days after he won his first ever medal when he placed second at the 2013 FIS World Championships.
In the 2014 Winter X Games, McMorris placed 2nd behind fellow Canadian Max Parrot in the Slopestyle event. McMorris' hopes of a 3rd consecutive gold medal were dashed when he tripped on the rails, crashed, and fractured a rib during the 3rd and final run of the event. Despite the injury, McMorris still competed in the Winter Olympics in Sochi. As McMorris arrived in Sochi, he said he felt remarkably good, stating, "The amount of progress I've made since I broke my rib on Saturday has been unexplainable. I didn't believe I could be on this path, going this fast. At the same time, walking around an airport is different than going snowboarding and taking impact."
At the Olympics, McMorris fell on his first qualifying run and failed to make it through directly to the final with his score on his second run. In the semi's he managed to put up one of the top four scores in order to qualify for the final. There, he again fell on his first attempt. McMorris' second run was good enough to put him on the podium and win bronze. It was a tough couple of weeks for McMorris with his injury and a flood of different emotions. He said that "To get on the podium is just a dream come true. I can't explain this. I've just been through a roller coaster over these last few weeks."
In February 2016 McMorris broke his right femur at Shaun White's Air + Style Big Air in Los Angeles. He attempted to do a frontside triple cork 1440, but in his landing his toe edge was caught which caused him to flip and break his femur. He recovered for a long time, starting at Fortius Sport & Health in Vancouver. He was back snowboarding in July 2016 in Australia with some friends. A series of four episodes following his recovery was produced by X Games, and aired in October 2016.
By the end of the 2016/17 season McMorris went boarding in the backcountry at Whistler, B. C. with some friends in late March 2017. He hurt himself very bad, and ended up at the hospital in Vancouver. His brother, Craig McMorris, spoke to The National shortly after the accident, and described what had happened "There was a fog [...] coming in and out, and Mark unfortunately drifted a little bit too far left, [...] we're hitting a feature, and he goes a little too far left and endes up in a clump of trees." McMorris suffered multiple conditions; a fractured jaw, a fractured left arm, a ruptured spleen, a pelvic fracture, rib fractures and a collapsed left lung. He underwent two succesfull surgeries. Mark is back on the board and hoping to compete For Team Canada during the 2018 Olympics Winter Olympics.
McMorris was born in Regina, Saskatchewan, Canada, and is the son of Saskatchewan provincial politician Don McMorris, a grain farmer, and Cindy McMorris, a nurse. His older brother, Craig McMorris, is also a professional snowboarder. McMorris attended Dr. Martin LeBoldus High School. McMorris also competed in wakeboarding before devoting himself to snowboarding. He has several tattoos, including one of sheaves of wheat representing the province of Saskatchewan.