Karen Iwata was born on May 13, 1998, in Miyagi Prefecture, Japan. Before joining the idol group AKB48, she began performing in musicals as a child in Sendai. Her career in voice acting began directly from her work as a member of AKB48, after she successfully auditioned for a role in an anime project centered on the group.

Iwata is known almost exclusively for a single, major voice role. In December 2011, she was selected from among AKB48 and its sister groups to provide the voice for Nagisa Motomiya, the central character of the television anime series AKB0048. She reprised this role for the second season, AKB0048 Next Stage, which aired in 2013. In the second season, her character also inherits the role of Atsuko Maeda the 14th. As part of the AKB0048 project, she became a member of the sub-unit NO NAME, which included other cast members from the show. The unit performed the series' theme songs, and their singles achieved top-three positions on the Oricon charts.

While her anime voice work is limited to the AKB0048 franchise, she has remained active as a live-action actress. Following her graduation from AKB48 in March 2016, she continued to pursue acting. She has appeared in several television dramas, including the action series Majisuka Gakuen 3 and the live-action adaptation of Saki, as well as feature films such as Solomon's Perjury and The Last Passenger. In June 2023, it was reported that she had left her talent agency.