After graduating from the University of Tokyo, Hamaguchi worked in the commercial film industry for a few years before entering the graduate program in film at Tokyo University of the Arts. His graduation film Passion was selected for the competition of the 2008 Tokyo Filmex. With Kō Sakai, he made a three-part documentary about survivors of the 2011 Tōhoku earthquake and tsunami, with Voices from the Waves being selected for the competition at the 2013 Yamagata International Documentary Film Festival, and Storytellers winning the Sky Perfect IDEHA Prize. His next film Happy Hour was first developed while Hamaguchi was an artist in residence at KIITO Design and Creative Center Kobe in 2013. It came out of an improvisational acting workshop he held for non-professionals, with many of the film's performers having participated in the workshop. The four lead actresses shared the best actress award and the film earned a special mention for its script at the 2015 Locarno Film Festival. Hamaguchi was also given a special jury award at the 2016 Japan Movie Critic Awards, as well as a best newcomer award in the film division of the Agency for Cultural Affairs's Geijutsu Sensho Awards that year. His Asako I & II was selected to compete for the Palme d'Or at the 2018 Cannes Film Festival.
