Sandén was born in Stockholm on 3 July 1992. She attended the Adolf Fredrik's Music School. She then attended the gymnasium school Rytmus Music High School, from which she graduated in spring 2011. Sandén practised and wrote music at Helges song studio in Gävle, where she also became a member of the band Helges All Stars with Alice Svensson, Amy Diamond, Zara Larsson, and her sisters Frida and Mimmi.
Sandén represented Sweden at the Junior Eurovision Song Contest in 2006, singing "Det finaste någon kan få". She came third, which remains Sweden's best result. She appeared in SVT's New Years' celebrations on 31 December 2006, which was broadcast live from Skansen. She also won the talent competition, Stellar Shots. She performed a duet with Magnus Carlsson in the song "Julens tid är här" ("Christmas time is here"), on his Christmas album Spår i snön in 2006.
Sandén also met Markoolio and sang with him during this period, on his new album called Snow at Christmas rock. In summer 2007, Sandén participated in Diggiloo along with Lasse Holm, Linda Bengtzing, Lotta Engberg, Tony Roche, Charlotte Perrelli, Thomas Pettersson, Jan Johansen, Niklas Andersson, Lasse Berghagen, Benjamin Wahlgren and Magnus Johansson.
She sang a duet with Ola Svensson on a Swedish version of "You Are the Music in Me" from the Disney film High School Musical 2. They also sang in the Lilla Melodifestivalen final ahead of the Junior Eurovision Song Contest in 2007. The previous year, Sandén was chosen to present the Swedish votes during the contest.
Sandén appeared on Så ska det låta on 11 April 2008 with Magnus Bäcklund. In summer 2008, she participated again in Diggiloo with Lasse Holm, Linda Bengtzing, Lotta Engberg, Thomas Pettersson, Måns Zelmerlöw, Nanne Grönvall, Stefan and Kim and Magnus Johansson. A compilation CD was released and included her performances of "Hallelujah" and "Gabriellas song". Her cover of Dea Norberg's song "Den underbara dagen du kommer hem" was used in advertisements for the fast food chain Sibylla in 2008.
Also in 2008, Sandén participated with Janne Schaffer, Jan Johansen, Johan Boding and Sara Löfgren in Sun, Wind & Water, a tribute to Ted Gärdestad. The same year, she and Brandur released a Swedish version of the song "Right Here, Right Now" from High School Musical 3. She participated in the celebrity dancing competition Let's Dance in 2010, which was broadcast on TV4; she was placed fourth.
In the 2009 Melodifestivalen, Sandén qualified for the grand final with her song "Så vill stjärnorna", but finished last with only two points, which were awarded by the international jury.
Sandén entered the Melodifestivalen again in 2012, again qualifying for the final with her song "Why Am I Crying", for which she came in fifth place. In August 2013, Sandén announced the premiere of her online clothing brand, 'My by Molly'. In November 2015 it was announced that Sandén was to participate in the Melodifestivalen for a third time in 2016. She performed in the fourth heat in Gävle on 27 February with the song "Youniverse", which she co-wrote with her boyfriend, Danny Saucedo, and John Alexis. She qualified from the semifinal to the final on 12 March. In the final she placed sixth with the juries and televoting, and also placed sixth overall.
Sandén has been in a relationship with Swedish singer Danny Saucedo since February 2013, and they got engaged in 2016. On 28 September 2012, she announced that she had been diagnosed with type 1 diabetes after collapsing while on a trip to Barcelona, Spain.2007: High School Musical 2 as Gabriella Montez (song)
2008: The Little Mermaid 3 as Ariel (voice and song)
2010: Tangled as Rapunzel (voice and song)
2012: Good Luck Charlie as Teddy Duncan
2011: Lemonade Mouth as Olivia White
2012: Blog Awards – Won "Blog rocket of the year"
2012: Rockbjörnen – Nominated "Female live artist of the year"–(lost to Loreen)