Machiko was born in Hachioji, Tokyo, Japan. She had humble upbringings and was raised to be a singer, though her talents were with acting. She was "discovered" after doing a play in Tokyo Center. From that night on, her life would forever change as she met many important figures in the world of Japanese television. Her mother died when she was a child; she was raised by her father. She had two brothers and a sister. One of her brothers died during the Second World War; the other is still alive. Her father died of cancer in 1991. In 1973, she went to study in Italy for two years. After taking jazz dance lessons for a number of years, her first roles were mainly radio and voice character roles. She made her debut on NHK's radio drama Chorinmura to Kurumi no Ki (1961), and later gained fame as the first voice actress to portray the lovable ghost Q-taro in Obake no Q-taro (TBS, 1965-1968). However, she is best known to fans of tokusatsu for her villainous roles in the Super Sentai franchise such as Queen Hedrian in Denziman and Sun Vulcan, she also appeared in Maskman portraying Tube Empire's field commander, Baraba's mother for one episode, as well as the evil sorceress Bandora in Zyuranger, better known to audiences in English speaking countries as Rita Repulsa in the American adaptation of Zyuranger, Power Rangers. In an odd twist, she soon found herself re-dubbing her own lines as Rita when Power Rangers was broadcast in Japan after the show became a surprise hit in America. Her final tokusatsu role was Magiel, Queen of the Sky Saints in Magiranger, which incidentally was one of her few non-villainous roles. In her memory, the producers of Power Rangers: Mystic Force used footage of Soga as Magiel to depict a reformed Rita Repulsa in the two-part final episode of Mystic Force, "Mystic Fate". Her very final role was in the PlayStation 2 game Space Sheriff Spirits as the voice and the "face" of Ankoku Ginga Jyoou (Dark Galaxy Queen), last boss and original character of this game inspired to the 80s Metal Hero series. Soga also played the voices of Cyborg 007 in the 1968 Cyborg 009 anime series and the sidekick Ball Boy in 1984 series Machineman. Machiko also ran her own shop, selling jewellery, antique clothing, and tapestries among other goods. In early August 2005, it was revealed that she had been diagnosed with pancreatic cancer about two years earlier. On the morning of May 7, 2006, she was found dead by a friend visiting her home. She was 68 years old. Her interment was in Fuchu, Tokyo's Tama Cemetery.