April Jeanette Mendez was born on March 19, 1987, in Union City, New Jersey. Her mother, Janet Acevedo, was a homemaker and later a home health aide, while her father, Robert Mendez, was an Automotive Engineer. She is the youngest of three children and of Puerto Rican descent. In describing her childhood, Mendez has said that her family struggled with poverty, mental illness, and drug addiction. They frequently moved between apartments, sometimes living in motels or their car when they could not afford rent.

Her brother's interest in World Wrestling Entertainment (WWE) influenced her ambition to become a professional Wrestler, which she cemented when she was twelve years old. She was inspired by WWE's female Wrestlers, especially Lita. In 2005, Mendez graduated from Memorial High School in West New York, New Jersey. She attended New York University's Tisch School of the Arts in New York City, where she majored in film and television production, until family and financial issues led to her dropping out six months into her studies. Afterward, she started working full-time to afford wrestling training. As an homage to her brother, who was in the U.S. Army, she once wore camouflage wrestling attire.

In March 2007, Mendez enrolled in a wrestling school close to her home, where she was trained by Jay Lethal. Around late 2007, she began to wrestle on the New Jersey independent circuit under the ring name Miss April.

She joined the New Jersey-based promotion Women Superstars Uncensored (WSU) in October 2008. After little success in her first months, Miss April formed a tag team with Brooke Carter, who together captured the WSU Tag Team Championship in February 2009. She also won the annual WSU/National Wrestling Superstars King and Queen of the Ring tournament alongside Jay Lethal two months later. In May, she left WSU upon signing with WWE and relinquished her championship.

Mendez paid US$1,500 to attend a WWE tryout camp in May 2009, where she was signed to a contract. She reported to Florida Championship Wrestling (FCW), WWE's developmental territory, and debuted under the ring name April Lee at a house show in late July. Shortly thereafter, her name was changed to AJ Lee.

Beginning in September 2010, AJ took part in the third season of NXT, a show that mixed professional wrestling and reality competition formats, with six female participants. Primo was assigned as her "WWE Pro", the role of her mentor in the storyline. She performed as a face (good guy) on the show and WWE adapted her real-life "nerd" fandom to her character. Throughout the season, a relationship between herself and Primo was teased, with them ultimately kissing on the finale. She also developed an on-screen friendship with fellow competitor, Kaitlyn. After surviving until the final three, AJ was eliminated from the competition on November 23.

AJ made her main roster debut on the May 27, 2011, episode of SmackDown as part of a tag team with Kaitlyn, referring to themselves as "The Chickbusters". Along with their mentor Natalya, they feuded with the trio of Alicia Fox, Tamina and Rosa Mendes for two months. In late July, AJ began a brief storyline on NXT Redemption as the girlfriend of Hornswoggle and feuded with Maxine. Meanwhile on SmackDown, she was attacked by Natalya, who turned on her protégé and allied with Beth Phoenix to form the Divas of Doom. AJ and Kaitlyn lost several matches to the Divas of Doom over the following months, causing tension between them, with Kaitlyn upset at their repeated losses.

In 2012, Mendez became the first female winner of WWE's annual Superstar Challenge, a video game tournament which consisted of 16 Wrestlers who competed in WWE '12 at WrestleMania XXVIII Axxess. This is recognized by Guinness World Records, as part of their Gamer's Edition.

AJ won a battle royal on April 22, 2013, to become the number one contender to Kaitlyn's WWE Divas Championship. Kaitlyn soon began a storyline where she received gifts from a secret admirer. On June 10, after setting up Langston to pose as the admirer, AJ revealed it to be a mind game, berated Kaitlyn and left her in tears. Six days later at Payback, AJ defeated Kaitlyn to capture her first Divas Championship. While she continued to mock Kaitlyn, they participated in the first ever in-ring contract signing for a Divas Championship bout. At Money in the Bank on July 14, AJ won their rematch but later in the show caused Ziggler to lose his own rematch for the World Heavyweight Championship. She hit his opponent, Alberto Del Rio, with her title belt to protect him from further head injury, prompting a disqualification. Angered by her unwanted action, Ziggler ended their relationship. She exacted revenge by costing him another match, then attacked him with Langston. On August 2, Langston was banned from attending her matches. However, AJ again retained the title from Kaitlyn, who was betrayed by Layla. Kaitlyn and Ziggler ultimately joined together to defeat AJ and Langston in a mixed tag team match at SummerSlam on August 18.

Mendez married former professional Wrestler Phil Brooks, better known as CM Punk, on June 13, 2014. They split their time between homes in Chicago, Illinois, and Milwaukee, Wisconsin.

In February 2015, Mendez spoke out via Twitter amidst widespread criticism from fans towards WWE's treatment of its female Wrestlers. WWE executive Stephanie McMahon had praised Actress Patricia Arquette for advocating women's rights and encouraged women to "use your voice". Mendez responded that, despite having generated record-selling merchandise and several top-rated segments, female Wrestlers in WWE receive only a fraction of the wages and screen time of their male counterparts. McMahon thanked Mendez for her public statement and her father, WWE chairman and CEO Vince McMahon, also acknowledged the issue.

A memoir written by Mendez, titled Crazy Is My Superpower: How I Triumphed by Breaking Bones, Breaking Hearts, and Breaking the Rules, was published by Crown Archetype on April 4, 2017. It covers her upbringing and career in professional wrestling. The book debuted at No. 10 on The New York Times Best Seller list for hardcover non-fiction. Upon its release, she stated that writing was "very much the next chapter" of her life and that she had begun work on a second book. As of December 2017, Crazy Is My Superpower was being developed into a television series, with Mendez serving as a Writer and executive Producer.