From the Super Mario Wiki
Yoko Shimomura (also credited Youko Shimomura) is a Japanese video game composer and musician. She is well known in the Mario franchise for composing the music for the Mario & Luigi series and Super Mario RPG: Legend of the Seven Stars.
Yoko Shimomura was born on October 19, 1967 in Hyōgo Prefecture, Japan. At the age of four, she started taking piano lessons and after being inspired by classical music, she began to play the piano randomly. After attending Osaka College of Music, she intended to become a piano instructor, but because she had been an avid gamer for many years, she decided to send some samples of her work to various video game companies. Afterwards, she began working for Capcom.
Shimomura later transferred to Squaresoft (now known as Square Enix). In 1996, she composed and arranged the music in Super Mario RPG: Legend of the Seven Stars, arranging music from Koji Kondo's Mario series music, and one track from Final Fantasy IV by Nobuo Uematsu.
After leaving Square Enix in 2002, she became a freelancer. She worked on the music for the Mario & Luigi series starting with Mario & Luigi: Superstar Saga in 2003 and has continued to compose the soundtracks for every entry to date. She has also created multiple arrangements for Super Smash Bros. Brawl, Super Smash Bros. for Nintendo 3DS / Wii U, and Super Smash Bros. Ultimate.