Trainor's lyrics frequently include themes of womanhood, body image, and personal empowerment; however, at times, they have been criticized for being antifeminist.

[6] At the age of 12, Trainor began performing as part of the cover band Island Fusion, which also included her aunt, younger brother, and father.

She attended Nauset Regional High School[6] and studied guitar, played trumpet, and sang in a jazz band for three years, and was a substitute cheerleader.

[26][27][28] It spent eight consecutive weeks atop the Billboard Hot 100 and was certified diamond by the Recording Industry Association of America (RIAA).

[29][30] Media outlets including Vice,[31] The Fader[32] and Complex[33] accused Trainor of cultural appropriation in "All About That Bass", The Seattle Times' Paul de Barros commented that Trainor affects an accent akin to that of a "young, urban, African American woman" in the song,[34] while its lyric "bringing booty back/Go ahead and tell them skinny bitches that" was criticized by Vox's Kelsey McKinney for dismissing people of smaller body types.

[49] Billboard ranked her as the fourth Top New Artist of the year[50] placing her 12th on their annual list of the most powerful people in music under the age of 21.

[76] She told MTV News the album was influenced by the music of Elvis Presley, Aretha Franklin, and Bruno Mars, and includes a collaboration between her and R.

[46] A performance of the song on The Tonight Show Starring Jimmy Fallon drew attention when Trainor fell to the floor while trying to catch hold of a microphone stand.

[89][90][91] The album received mixed reviews, resulting in a Metacritic score of 60 based on 10 of them, and debuted at number three on the US Billboard 200; it was certified platinum in the US for sales in excess of a million units.

[97] She co-wrote multiple songs that year, including Jennifer Lopez's "Ain't Your Mama" and "You Gotta Not" from Little Mix's album Glory Days.

[98][99] On February 24, 2017, Trainor released the single "I'm a Lady", which she recorded for the film Smurfs: The Lost Village, in which she also voiced a character called Smurfmelody.

[100] In December 2017, Fox announced Trainor as one of the judges on the show The Four: Battle for Stardom, along with Sean Combs, DJ Khaled and Charlie Walk.

[102] Trainor competed against singer Shania Twain in an episode of TBS's show Drop the Mic, which aired in January 2018; both were declared winners.

[108] In collaboration with Sigala and Ella Eyre, Trainor released the single "Just Got Paid" which features American rapper French Montana.

[112] Trainor launched The Salvation Army's 128th annual Red Kettle Campaign on November 22, 2018, performing at a Dallas Cowboys and Washington Redskins game.

[122] In September 2019, ITV announced Trainor would join will.i.am, Sir Tom Jones, and Olly Murs as a coach for the ninth season of The Voice UK, which premiered in 2020.

[125] Trainor wrote enough material for four albums while trying to adapt to new trends in the music industry,[126] Treat Myself was released on January 31, 2020, along with its third single "Nice to Meet Ya" which features Nicki Minaj, to mixed reviews which resulted in a Metacritic score of 51 based on four of them.

[144] Trainor featured on Jason Derulo's song "Hands on Me" in October 2023, with Paris Hilton making an appearance in its music video.

[152] The music video for Timeless's second single, "To the Moon" (2024), featured appearances by Sabara, their son, Niecy Nash, and the influencers Chris Olsen and Brookie and Jessie.

[164] Trainor has also cited artists like the Backstreet Boys,[165] Ariana Grande, Jason Mraz, and T-Pain as influences on the "upbeat and happy" nature of her music.

[166] Aretha Franklin and Elvis Presley influenced Thank You, whereas its lead single "No" draws inspiration from Britney Spears, Christina Aguilera, Destiny's Child, and NSYNC.

[171] Her debut EP Title is composed of throwback-style sound and "1950s doo wop-inspired songs that straddle the line between modern R&B and melodic pop".

[175] Her hook-laden songwriting style has been likened to that of Brill Building composers such as Gerry Goffin, Carole King, Ellie Greenwich, and Jeff Barry;[176] while the sound of her debut EP has been compared to that of American indie pop singer-songwriters Jenny Lewis and Neko Case,[177] and the "retro-girl-group" sound of her debut single to that of 1960s singers such as Betty Everett and Eydie Gormé.

[187] Billboard's Ashley Lee wrote that since receiving controversy for the lyrics of "All About That Bass", Trainor began writing "more inclusive" songs about body positivity, citing "Me Too" and the Thank You track "Woman Up" (2016) as examples.

[196] MTV News' Carvell Wallace wrote that the singer has charismatic vocals, her relatability is part of her appeal, and one can "imagine a drunken bachelorette party having a transcendent sing-along to [her music] in the back of a rented stretch Hummer".

[187] In March 2015, Trainor partnered with plus-size retailer FullBeauty Brands as a consultant for the creation of clothing for women with varying body types.

[200] According to Billboard's Jada Yuan, Trainor's image is defined by her "curves" though she is not "curvy" like Nicki Minaj and Kim Kardashian but "she's not model-thin like many other stars".

[212] On January 30, 2023, Trainor announced she and Sabara were expecting their second child together,[213] and confirmed in April 2023 on The Kelly Clarkson Show that she was pregnant with a boy.

[215][216] Trainor has partnered with the American Cancer Society,[217] and made public appearances in charity events such as We Day California,[218] as well as The Hollywood Reporter's 24th annual Women in Entertainment Breakfast.

[220] During the COVID-19 pandemic, Trainor donated funds to feed healthcare professionals in Greater Hartford and pledged money for restaurants in Connecticut to provide 50 lunches to local hospitals daily for five days.