When Maroney was 9 years old, she started training at Gym-Max in Costa Mesa, California. In 2012, she said, "I realized that I had a dream that I wanted to go to the Olympics and I wanted to go as far as I could go. I thought I should probably move to a new gym and start thinking about the Olympics."
After winning the silver medal in the vault finals, Maroney was photographed on the medal podium while giving a brief look of disappointment with her lips pursed to the side. The image became an Internet phenomenon that was sparked by a tumblr blog called "McKayla is not impressed". The image went viral after it was Photoshopped into various "impressive" places and situations, e.g. on top of the Great Wall of China and standing next to Usain Bolt. Maroney stated that she found the Internet meme "kinda funny". Maroney later poked fun at the meme on various occasions including appearances on Late Show with David Letterman, The Colbert Report, Dancing with the Stars: All-Stars, Extra, and most notably when she and the rest of the U.S. Gymnastics team met United States President Barack Obama at the White House in November 2012. She and President Obama posed for a photo together, both making the same pursed-lips expression. The podium picture made the top spot on Yahoo's list of "Most Viral Photos" of 2012.
In September 2012, Maroney was injured during an uneven bars dismount while performing at the Kellogg's Tour of Gymnastics Champions in Ontario, California. An MRI revealed that she had fractured the tibia in her left leg. The injury occurred shortly before teammate Aly Raisman injured herself performing on the same bars. On September 13, 2012, Maroney had two screws inserted to repair the fractured left tibia. She wore an immobilizer to keep the leg straight until doctors determined that she was ready to resume limited walking and rebuilding range of motion. Maroney returned to The Kellogg's Tour on October 4, 2012 in Oklahoma City on light duty. She was seen during the finale wearing her leg and foot splints and also at the VIP Meet and Greet session after the event.
Maroney made her acting debut in the CW television series Hart of Dixie on November 20, 2012. She played Tonya, a friend of Rose in the episode "Baby, Don't Get Hooked on Me". That appearance turned into a recurring role, with Maroney appearing in six episodes of the series.