Rintaro was born and raised in Hyogo Prefecture, Japan. He is a qualified paediatrician in both Japan and the United Kingdom, with extensive experience in social policy development across Japan, Australia, the United Kingdom, and in low- and middle-income countries. He has authored over 300 scientific publications. He served as a technical adviser on population policy at the United Nations Population Fund until 2023 and was elected Mayor of Takarazuka City in Hyogo Prefecture in April 2025.