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 tapestry among other goods.

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 sister died in childbirth, and her father died of cancer in 1991. In 1973, she went to study in Italy for two years.

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 Fuchū, Tokyo's Tama Reien Cemetery.

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 actor to portray the lovable ghost Q-taro in Obake no Q-tarō (TBS, 1965-1968).