In 2004, Mendler got her first acting role in the animated Indian film The Legend of Buddha, in which she portrayed Lucy. When she was only 13 years old, she got an acting role as a guest star on the soap opera General Hospital. She portrayed the dream child of character Lulu Spencer, in which the two have an argument on Mendler's character's birthday. The scene, lasting just under a minute, is later revealed to be a dream. That same year, Mendler was the voice of the character Thorn in the video game Bone: The Great Cow Race, which was based on the Bone comic series.
In 2007, Mendler made her film debut in the film adaption of the Alice series, titled Alice Upside Down. Mendler starred alongside Disney Channel actress Alyson Stoner and Lucas Grabeel. Bridgit portrayed the antagonistic role of Pamela, who is the rival of Stoner's character, Alice. For the film's soundtrack, Mendler provided backing vocals on the song "Free Spirit", performed by Stoner. The film was released straight to DVD on October 6, 2007. In 2008, it was announced that Mendler would portray the role of Kristen Gregory in the film adaption of the popular teen novel series The Clique. Mendler had the role of Kristen, a girl who attends OCD on a scholarship, and works hard to keep her good grades. The Clique was released straight to DVD in Fall of 2008, her second film to be released in this format. In 2007, Mendler had begun working on a film with actress and singer Lindsay Lohan titled Labor Pains, which kept being pushed back due to various conflicts and problems. Though initially slated for a theatrical release, the film did not receive one in the US and was instead released as a TV film on ABC Family. The film did, however, receive a theatrical release in countries such as Russia, Romania, Spain, the U.A.E., Ecuador, and Mexico. The film drew 2.1 million viewers, a better-than-average prime-time audience for ABC Family, and per the network, was the week's top cable film among coveted female demographic groups.
Beginning in 2009, Mendler became a recurring character in the Disney Channel series Wizards of Waverly Place, alongside Selena Gomez and David Henrie. Mendler portrayed the role of Juliet Van Heusen, a vampire who later forms a romance with Henrie's character Justin Russo. Mendler would go on to appear in eleven episodes total for the series, spanning from 2009 to 2012 when the series officially ended. She had a supporting role in the film Alvin and the Chipmunks: The Squeakquel. Also in 2009 Mendler auditioned for Sonny with a Chance, but Demi Lovato was chosen as Sonny Monroe.
In 2010, Mendler became the star of the Disney Channel Original Series Good Luck Charlie, centering around a teenage girl who makes videos for her baby sister to watch as she gets older. Adam Bonnet, Senior Vice President of original programming for Disney Channel, says Disney Channel executives "just fell in love with" Bridgit Mendler, who stars as the series' protagonist, a teenager named Teddy Duncan. "She has all the attributes of a Disney star", said Bonnet. Mendler first heard about Good Luck Charlie in late November 2008. After several rounds of auditions and cast reads, she finally secured the part in January 2009. Mendler recorded the theme song for the series, titled "Hang In There Baby". There series premiered on April 4, 2010, and has since been met with a positive critical reception and viewership. The series premiered to 4.6 million viewers, making it the highest-rated series premiere for a Disney Channel Original Series since The Suite Life on Deck in 2008, and the week's highest-rated cable program. Maclean's reported that overall, the first season "has been doing about as well as Disney’s more successful shows – The Suite Life and so on". The series has, as of November 2012, aired three seasons, and was renewed for a fourth in July 2012.
In 2011, she starred as Olivia White, the lead role in the Disney Channel Original Movie, Lemonade Mouth. On the film's plot line, Mendler stated "You have the stories of these five separate kids who come together and form a band. From the first song to the last the group grows and progresses and the music reflects that." Mendler performed numerous songs for the film's soundtrack, which was released on April 12, 2011 by Hollywood Records.
Lemonade Mouth was watched by 5.7 million viewers on its premiere night. ranking as the No. 1 TV Telecast among Kids 6–11 (2.3 million/9.4 rating) and Tweens 9–14 (2.1 million/8.5 rating), and cable's No. 1 original movie of 2011 among Total Viewers. With DVR viewing included, its total was 7.1 million viewers. In an interview with Kidzworld Media, Mendler confirmed that there will not be a sequel to Lemonade Mouth, commenting: "There’s not [going to be a sequel to Lemonade Mouth] unfortunately. We had such a great experience working on the movie, and they tried to figure something out for a sequel, but everyone at Disney felt like the movie had completed its story in the first movie. It was a great experience, and I loved working with the cast members and still see them frequently."
In 2011, Mendler had the supporting role of Appoline in the straight to DVD film Beverly Hills Chihuahua 2, starring George Lopez and Emily Osment. The movie had a mixed critical reception, but went on to sell nearly 30 million copies by the end of 2012. Mendler recorded the song "This Is My Paradise" for the film, which was released as a promotional single on January 11, 2011. The video of the song was recorded in Beverly Hills, California, in November 2010 and directed by Alex Zamm. On March 31, 2011, it was confirmed that Mendler had signed with Hollywood Records, and had begun working on her debut album. Later that Winter, Mendler also appeared in the Disney Channel Original Movie Good Luck Charlie, It's Christmas!, which premiered on December 2, 2011 on Disney Channel, almost ten years after Disney Channel's last Christmas-themed original movie, 'Twas the Night in 2001.
In 2012, she guest starred in the television series House as Callie Rogers, a homeless runaway teenager with a mysterious illness. She voiced the lead role of Arrietty in the American English dub of The Secret World of Arrietty and recorded a song "Summertime", which was featured in the movie's closing credits.
Mendler released The first single with Lemonade Mouth's soundtrack, titled "Somebody", was released on March 4, 2011. It went on to sell over 6,000 copies in its first week of release, according to Nielsen SoundScan. It went on to peak at number 89 on the US Billboard Hot 100 chart and at number 12 on the US Billboard Top Heatseekers chart. The soundtrack's second single, "Determinate", charted in numerous other countries and peaked at number 51 on the Billboard Hot 100.
The song "I'm Gonna Run to You" was co-written and performed by Mendler, and was also featured in the film. It was released on iTunes on November 12, 2011. It was watched by 7 million viewers, the premiere also delivered 3.3 million Kids 6-11, 2.4 million Teens and 1.4 million Adults 18-49. The movie became no. 1 live-action cable movie of 2011 in total viewers and the no. 1 live-action scripted telecast across all TV in Kids 6-11. Mendler later co-wrote and sang the Disney's Friends for Change Games anthem called "We Can Change the World".
In the Summer of 2012, Mendler confirmed that the title of her official debut single was "Ready or Not". The single premiered on August 3, 2012 and was released for digital download on August 7, 2012. It was released for radio airplay on August 21, 2012. The song was written by Mendler herself, Emanuel "Eman" Kiriakou and Evan "Kidd" Bogart and peaked at number 49 on the US Billboard Hot 100 chart. It is also reached number 26 on the US Pop Songs. It also peaked on 48 on the Canadian Hot 100 and 12 on the New Zealand Singles Chart, becoming her first Top 20 hit internationally. It has sold 21,000 copies in its first week according to Nielsen SoundScan. As of January, 2013 it was certified Gold. Mendler's debut album, Hello My Name Is..., was released on October 22, 2012. It debuted at number 30 on the Billboard 200 and at number 20 on the Digital Albums chart. By its first week of release, the album had sold an estimated 18,000 copies. It debuted and peaked at number 17 on the Polish Albums Chart, making it the first country to chart outside the US. She also release as promotional single "Forgot to Laugh" and "Top of the World". Mendler performed her hit song, "Ready or Not" on the second season of The X Factor USA, on December 13, 2012.
Mendler ventured on her first headlining tour, Bridgit Mendler: Live in Concert. It is the debut concert mini-tour by Mendler herself. Marked as the band's headlining tour, it supported her first studio album. The tour primarily reached the United States and Canada. Mendler's tour playing at state fairs and music festivals in United States and Canada and Jingle Ball's concerts series. "Rocks at My Window" and "Hold On for Dear Love" are the only songs that were never performed. In 2013 "Hurricane" was announced as second single. The song received critical acclaim from music critics and praised heavily on Mendler's rapping skills, which have been compared to Cher Lloyd and Karmin. For the week ending October 27, 2012, the album version of "Hurricane" debuted at number 194 on the South Korean International Singles Chart, making it the second song to chart in the country. It also peaked at number 4 at the Ukrainian Charts, her second top 5 hit there. As of May 4, 2013, the song has sold 100,977 digital copies in the US and debuted at number 3 in the US Billboard Bubbling Under Hot 100 chart and peaked at number 1.
In an interview with Elvis Duran in September 2012, Mendler admitted that she is dating her Good Luck Charlie co-star, Shane Harper. When asked, "Is it Spencer from Good Luck Charlie?" that she is dating. She responded, "It wasn’t one of those…'you meet on a set and date instantly', it took two years." The two have been dating since May 2011. Mendler later played Harper's girlfriend in the music video for his 2012 song "Rocketship". She is currently taking a class at the University of Southern California.