Maroney was born in Aliso Viejo, California, to Mike and Erin Maroney, on December 9, 1995. Her father was a quarterback at Purdue University, and her mother was involved in figure skating and high school sports. She is Irish-Catholic. She has two siblings, Tarynn and Kav, and was homeschooled in order to train as an elite gymnast.
"When I was younger," Maroney said, "I would be watching Tarzan and running around on all fours. My mom was like, 'I need to put this child in gymnastics. She's crazy.' I was technically in gymnastics at the age of 2 and always felt comfortable in the gym."
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."
About her first Olympic memory, she said, "I remember watching Carly Patterson. She was one of my favorites. I loved her floor routine. I went to Olympic Trials that year (2004) with Kyla [Ross]. I remember seeing the gymnasts on the floor and I loved all of their leotards. I thought they were so little and so amazing. I thought, how am I going to stay this little? I was worried that I was going to be really tall because my dad is 6'3" and my mom is 5'2", so I didn’t know which one I was going to be."
In August, Maroney competed at the Visa Championships in Dallas, Texas. She placed twenty-seventh in the all-around with a two-day combined score of 104.800 and third in the vault final with a score of 29.900. When asked about the first time she performed an Amanar in competition, Maroney said, "The first time I did it was at Visa Championships and I was just thirteen years old. I was just really little and I didn't really know what was going on. But I just did it, and I was just very happy that I landed on my feet."
In July, Maroney competed at the CoverGirl Classic in Chicago, Illinois. She placed seventh in the all-around competition with a score of 55.650.
In August, Maroney competed at the Visa Championships in Hartford, Connecticut. She placed third in the all-around competition with a two-day combined score of 113.250. In event finals, she placed first on vault scoring 32.000, seventh on balance beam scoring 27.850, and fourth on floor scoring 28.400.
In September, Maroney competed at the Pan American Championships in Guadalajara, Mexico. She contributed a vault score of 15.933, a balance beam score of 13.866, and a floor score of 14.500 towards the American team's first-place finish. In event finals, she placed first on vault with a score of 15.387 and first on floor with a score of 14.225.
In March, Maroney competed at the City of Jesolo Trophy in Jesolo, Italy. She won the all-around competition with a score of 57.850.
In July, Maroney competed at the CoverGirl Classic in Chicago, Illinois. She placed sixth on balance beam scoring 14.550 and fifth on floor scoring 13.450.
In August, Maroney competed at the Visa Championships in Saint Paul, Minnesota. She placed second in the all-around competition with a two-day combined score of 115.150. In event finals, she placed first on vault scoring 30.700, seventh on balance beam scoring 27.650, and fifth on floor scoring 28.150.
In October, Maroney competed at the 2011 World Artistic Gymnastics Championships in Tokyo, Japan. She contributed a vault score of 16.033 and a floor score of 14.566 towards the American team's first-place finish. Maroney won the vault final with a score of 15.300. She told International Gymnast Magazine, "For every competition, I just went in there with confidence in myself and in my team, and each day it felt like I gained more and more experience."
Maroney upgraded her second vault to a round-off, half-on, full-twisting front layout. "I've done it at two [U.S. team training] camps so far, and it's going pretty well," Maroney said. Her first vault remained an Amanar.
In March, Maroney competed at the City of Jesolo Trophy in Jesolo, Italy. She placed fourth in the all-around competition with a score of 57.950.
In May, Maroney competed at the Secret U.S. Classic in Chicago, Illinois. She placed first on vault with a score of 16.100 and tenth on balance beam with a score of 13.800.
In June, Maroney competed at the Visa Championships in St. Louis, Missouri. After the first day of competition, she was placed seventh in the all-around with a score of 58.700. During warmups for the second day of competition, she fell on a tumbling pass on floor landing on her back. Maroney was taken to the hospital and diagnosed with a minor concussion and nasal fracture. She took a week off from training and successfully petitioned to compete at the Olympic Trials.
At the beginning of July, Maroney competed at the Olympic Trials in San Jose, California. After the first day of competition she said, "I'm thankful that I'm even here. I knew I had to hit vault because it's the most important event for me to make the Olympic team." Though she had falls on both bars and beam on day one, and a major loss of formation on bars during day two, she placed seventh in the all-around with a two-day combined score of 117.650. In event finals, she placed first on vault scoring 31.700 and fifth on floor scoring 29.700. Afterwards, Maroney was chosen as a member of the team that was sent to the 2012 Summer Olympics. She said, "This is the best feeling – it's just so amazing. I'm in shock right now. I think this will be the happiest I'll ever be in my life. It's the absolute best feeling knowing that I trained so hard for this and all the hard things that I have accomplished and now to be on this team. These girls are my best friends. My team mate Kyla Ross, we have been best friends since we were six years old so it's just absolutely amazing to be going to the Olympics." When asked about feeling pressure to win gold on vault at the Olympics, she replied, "If I do my job I know I can get the gold medal. That's what they picked me to be on the team for, so I need to be confident about that."
Maroney was featured on the cover of Sports Illustrated with the rest of the USA Women's Olympic Gymnastics team on the July 18, 2012, issue of "Olympic Preview". This marked the first time an entire Olympic gymnastics team had been featured on the cover of Sports Illustrated.
At the end of July, Maroney competed at the 2012 Summer Olympics in London, England. During training she aggravated a previous bone break in the big toe of her right foot but she still competed on vault, though she did not take part in the floor routine as she had hoped. "Bad things happen, you just have to make the best of it," Maroney said. "It does hurt. It’s broken. How is it not going to hurt? I just try to ignore it and I have worked so hard to be here I can ignore the pain for a little bit." She helped the American team qualify in first place, and individually qualified in first place to the vault final with a score of 15.800.
Along with Jordyn Wieber, she is credited for changing the team's nickname from the "Fab Five" to the "Fierce Five". While on the bus to a training session, they decided to change the name because the "Fab Five" was already used for a basketball team associated with the University of Michigan as well as being associated with the musical group Duran Duran. They started searching on their phones for words that started with the letter F that described the team. The top choices were feisty and fierce. Maroney and Wieber opted for "fierce" as they said it described their floor routines, and the rest of the team concurred.
In the team final, she contributed a vault score of 16.233 toward the American team's first-place finish, which was the highest score in the competition. She also scored an execution average of 9.733, the highest execution score at an Olympics or world championship under the new scoring system for women's gymnastics. About the rest of the team final, Maroney said, "I just wanted them to do so well and I'm just so proud of them. I was screaming and yelling. That last event on floor was really exhilarating, but amazing at the same time. I don't think I will ever, ever forget that."
In the vault final, Maroney won a silver medal with a score of 15.083. She performed an Amanar for her first vault but fell on her second. This fall ended her 33-vault hitting streak in competition. Even with the fall, Maroney still came in second. She said, "I didn't get my full block, my hands didn't really touch the vault. I still walked out with a silver medal and I'm happy about that. I'll have to watch the video and figure out what happened. It's really sad that I had to fall on that vault but I'm glad I won a silver medal. I know I can do better vaults but I also know I didn't deserve the gold medal because I fell on my second vault. It happens. It's gymnastics. You can't always be perfect. Sometimes things don't go as planned."
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 The 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.
On January 12, 2013, Maroney served as one of the judges for the Miss America pageant.
She participated in the Thirty Seconds to Mars music video "Up in the Air".
In 2013, Maroney signed a sponsorship agreement with Adidas Gymnastics.
In July 2013, Maroney made her return to gymnastics at the 2013 Secret U.S. Classic, where she placed first on vault with a score of 15.425 (15.600 + 15.250) and third on floor with a score of 14.350, posting the highest execution score on that apparatus. This was her first event since the 2012 London Olympics. In an interview after she finished, Maroney said she hoped to upgrade both her vaults by the 2016 Olympics, as well as her tumbling on floor.
At the P&G Championships in August, Maroney competed on vault and floor exercise. She won gold medals in both events.
On September 15, Maroney was one of four gymnasts named to the United States team for the 2013 World Artistic Gymnastics Championships in Antwerp, Belgium. She was originally put on the team to compete on only two events—vault and floor, her strongest ones—but was later named to compete as an all-around gymnast alongside Simone Biles and Kyla Ross after performing extremely well during podium trainings. Teammate Brenna Dowell was named as an alternate instead, as there could only be three gymnasts competing on each event. She scored a total all-around score of 57.149, placing 6th overall, but due to the two-per-country rule, she could not compete in the all-around final or the floor final (14.333), or qualify as third reserve to the uneven bars final (14.3). On floor, Maroney would have qualified over teammate Kyla Ross but due to her music being over time by one second she incurred a 0.1 deduction, putting her in the 6th place with same score as Ross. Due to Maroney having lower execution, albeit having higher difficulty, Ross progressed to the final. In the vault final, she successfully defended her title, winning the gold medal with a score of 15.724. It was the last competitive routine of Maroney's career. She was the first American woman to ever defend a world title on vault.
In March 2014, Maroney underwent surgery on her knee. Maroney was featured on the cover of the April issue of Inside Gymnastics. In International Gymnast Magazine, she said that she wanted to upgrade both of her vaults.
On August 31, 2014, allegedly underage nude images of Maroney were published as part of the 2014 celebrity photo leaks.
In March 2015, Maroney stated in a YouTube video that she struggled with adrenal fatigue following the 2013 World Championships.
On February 24, 2016, popular gymnastics podcast GymCastic released a candid 49-minute-long interview with Maroney regarding her health issues preceding and following the 2012 Olympic Games. In the interview, she announced her retirement from competitive gymnastics—this came two days after fellow Fierce Five teammate Kyla Ross' retirement.
In May 2017, Maroney posted a video of herself dancing in a thong on Instagram, which Maroney confirmed that she posted herself and wasn't hacked. It led to criticism from those who saw her as a role model to young girls, which she later stated on Twitter that she isn't despite public perception of her.
Maroney made her acting debut in the CW television series Hart of Dixie on November 20, 2012. She played Tonya, a friend of Rose's 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.
In December 2013, Maroney appeared on an episode of the Fox TV show Bones called "The Spark in the Park" (season 9), as a gymnast named Ellie who is a suspect in the murder of a fellow gymnast.
In summer 2016 Maroney announced that she would be making a singing debut with a single named "Ghost".