Mizushima is bilingual, being fluent in English and Japanese. He spent his whole elementary school years (1990 to 1996) in Zurich, Switzerland. He was bullied and discriminated as a child at International School in Switzerland. When he came back to Japan, he attended the junior high school and the high school of Toin Gakuen, and belonged to the soccer club of the school. When he was in the twelfth grade, he worked as a regular midfielder in The 81st All Japan High School Soccer Tournament.
In 2006, Mizushima became a main character in the series Kamen Rider Kabuto and acted in the lead role in the film Kamen Rider Kabuto: God Speed Love.
Beginning in 2007, Mizushima began taking on bigger roles in more high-profile productions, such as the supporting role of the girl-crazy Minami Nanba in Hanazakari no Kimitachi e and the main role of Soshi Asamoto in Zettai Kareshi in 2008. Both dramas were among the most popular dramas of Japan at the time of their airing. Mizushima also provided the voice of Bruce Banner in the Japanese dub of The Incredible Hulk. Mizushima took the role of Jiro Mori in the 2008 TV drama Room of King.
He married Japanese pop singer Ayaka on February 22, 2009. In December 2014, the couple announced through their agency that they are expecting their first child. In June 2015, Mizushima announced via Instagram that he had become a father. The couple has a daughter together.
In September 2010, Mizushima left his talent agency, Ken-On, to create an own independent agency called A stAtion. Several weeks later, on 31 October 2010, it was announced that Mizushima had won the 5th Poplar Publishing Grand Prize for Fiction and a cash prize of 20 million yen for his debut work, titled Kagerou, written under the pen name Satoshi Saitō (齋藤 智 Saitō Satoshi) but he decided to decline the 20 million yen prize money. The book was published in December 2010. In the two and a half years of Oricon's book charts, Kagerou recorded the third highest first-week sales, behind J. K. Rowling's Harry Potter and the Deathly Hallows (1.191 million) and Haruki Murakami's 1Q84 Book 3 (398,000).
In 2014, Mizushima starred as Sebastian Michaelis in the film Black Butler, in which he also serves as the co-screenwriter and co-producer. The film will be shown at Toronto Japanese Film Festival and Brussels International Fantastic Film Festival 2014.
In August 2015, Mizushima has been cast to appear in the 5th season of HBO's Girls, which aired on 2016. This is his first international TV appearance.
