Ratchanok won her first individual International title in 2009, while she was only 14, by winning the Vietnam International Challenge. She made history by becoming the youngest-ever champion at the BWF World Junior Championships at 14 when she triumphed in Malaysia. She also reached SEA Games 2009 Badminton WS final but lost to her compatriot Salakjit Ponsana.
In 2010, at the age of 15, she successfully defended her title at the World Junior Championships in Mexico. She won two back-to-back Grand Prix tournaments by winning YONEX-SUNRISE Vietnam Open Grand Prix and Indonesia Open Grand Prix Gold. In 2010 Guangzhou Asian Games, she won a silver medal as a member of the women's team. In the final, she lost to Wang Xin, at that time world number 1, 22-20 17-21 14-21.
In 2011, she became the most successful player ever in individual events at the BWF World Junior Championships, winning the women's singles title for the third straight time in Taiwan. She won YONEX-SUNRISE Syed Modi Memorial India Open and was also a member of the women's team that defeated Indonesia in the final at the 2011 SEA Games. At the BWF World Championships, she was the only player to take a game off of the eventual champion, Wang Yihan.
In 2012 Ratchanok, at 16 years of age, was awarded the Best Female Athlete Award in Thailand after winning the world junior title for three successive years. Ratchanok's biggest goal is to win the Olympic gold medal. However, at 2012 London Olympics quarter-final match with Wang Xin, despite leading 21–17 and 16-9 in the second game, she failed to close the match and eventually lost 21–17, 18–21, 14–21. She reached the finals of the SCG Thailand Open 2012 but lost to Saina Nehwal 19–21 21–15 21–10 in the finals. She entered the finals of a Super Series tournament for the first time in 2012 China Open Super Series Premier but lost to Li Xuerui 12-21, 9-21. She qualified for the Super Series Finals and lost in the semi-finals. She finished the year as world number 9.
2013 was one of Ratchanok's golden years. She reached the finals of the 2013 All England Open Badminton Championships, losing to Tine Rasmussen 14–21, 21–16, 10–21. Despite her loss, she is still the youngest singles finalist ever at the All England tournament. She finally won her first Superseries tournament by beating Juliane Schenk 22-20, 21-14 in Yonex Sunrise India Open 2013 to become the youngest ever Superseries winner, with the age of 18 years 2 months 22 days (She hold this record for 6 months until Akane Yamaguchi won Japan Open 2013 with the age of 16). She again reached the finals of the SCG Thailand Open 2013. This time she won the title, beating Busanan Ongbumrungpan 20-22, 21-19, 21-13 to become the first Thai ever to win the women's singles title at the Thailand Open since first held in 1984.