Louisa was born on 11 January 1998 in Essex to David Johnson and Lisa Hawkyard, and has one younger brother, Fynn Johnson. Her father is a construction worker from Essex and her mother is a driving instructor from Chafford Hundred. She was raised in Thurrock.

In 2015, Johnson auditioned for the twelfth series of The X Factor, singing American pop music family group The Jackson 5’s “Who’s Lovin’ You”. She picked this record because “it is a really really hard song to sing, but I wanted to make a good impression”. She received a standing ovation from all four judges and the audience afterwards and progressed through to “bootcamp”. At the first challenge at “bootcamp”, she sang “Proud Mary” with 4th Impact (formerly known as 4th Power), Sherilyn Hamilton-Shaw, Neneth Lyons, Jasmine Leigh Morris and Andre Batchelor, and was sent through to the next challenge. She sang “Lay Me Down” in the second challenge and made it through to the six-chair challenge.

Following the eliminations of Weathers in the second week, Michael in the third week and Murray in the semi-final, Johnson advanced to the final along with Reggie ‘n’ Bollie and Ché Chesterman, additionally becoming the final female in the competition. On 12 December, she duetted with her mentor, Ora. The following night, she sang her winner’s single, a cover of “Forever Young” by Bob Dylan. On 13 December 2015, she won The X Factor, becoming the first winner in the Girls category since Alexandra Burke in 2008, and also the youngest winner ever, overtaking Joe McElderry, who was 18 years old when he won in 2009.

After winning The X Factor, Johnson released “Forever Young”. The day after her victory, she plugged the single on The X Factor judge Nick Grimshaw’s radio show, Good Morning Britain, Lorraine and This Morning, consecutively. Her version of the song entered the UK Singles Chart on 18 December 2015 at number 9, and became the lowest ever position on entry for a debut single from an X Factor winner. This can be attributed, in part, to the movement of the chart day from Sunday, when she was announced as the winner, to Friday, resulting in her single only receiving four days’ sales. The single dropped to number twelve the following week becoming the lowest ranking X Factor winner’s song ever on the Christmas Day chart. Overall, the single is the lowest charting X Factor winner’s single to date as all other winner’s singles managed to top the chart or at least reach the top five. On 18 December 2015, Johnson performed “Forever Young” on the 2015 Text Santa broadcast.

In May 2016, Louisa released a single with British band Clean Bandit called “Tears”. The song was a commercial success, reaching the top five in Scotland and the UK, and charting in many other European countries. It was also certified platinum by the British Phonographic Industry (BPI). In June, she performed “Tears” with Clean Bandit and a new version of the song “Over and Over Again” with Nathan Sykes at the Summertime Ball. She co-headlined the concert the following year, and appeared as a guest performer in 2018.

Louisa released her second solo single, “So Good”, in November 2016. The single peaked at number 13 in the UK, and received a gold certification from BPI. The following month, Johnson co-headlined both nights of the annual Jingle Bell Ball at London’s O2 Arena; she co-headlined the concert for second consecutive year in 2017. Her third single, “Best Behaviour” was released in March 2017. It charted at number 48 in the UK, and was certified silver by BPI. In 2017, she also featured on Olly Murs’ single “Unpredictable”, which reached number 32 on the official UK singles chart and received a silver certification from BPI. In March 2018, Louisa released her fourth single, “Yes”, a collaboration with the American rapper 2 Chainz, which peaked at number 65 in the UK.

On 13 July 2018, Johnson announced via social media that she had left Syco. She also announced the release of her new single for early August with Ministry of Sound. The single “Between You & Me”, recorded in collaboration with One Bit, was released on 10 August 2018. Later that month, she was featured on Mars Moniz’s single “999”. In 2019, she collaborated with Sigma on the single “Here We Go Again” which charted in the UK at number 98. She also featured on Kream and Eden Prince’s single “Ain’t Thinkin Bout You”, and released “Ain’t Got You” with Steve Void in 2019.

In January 2020, Johnson announced a collaboration with fitness clothing brand USA Pro. Following the announcement, she released “Like I Love Me” to promote the collaboration.