Ado Onaiwu was born on 8 November 1995 in Kamikawa, Hokkaido, to a Nigerian father and a Japanese mother. After developing through Renofa Yamaguchi he signed with Urawa Red Diamonds, lifting the AFC Champions League in 2017 as a young squad member. Loan spells at Oita Trinita (2019) and Yokohama F. Marinos (2020) preceded his permanent move to Yokohama F. Marinos in 2020.
In summer 2021 Onaiwu took on the European challenge with Toulouse FC in France's Ligue 2. He was an immediate hit — 38 league appearances with 10 goals in his debut season — and helped Toulouse to the Ligue 2 title in 2021–22 and the Coupe de France in 2022–23. After the 2022–23 season he joined Auxerre, where he again won promotion (Ligue 2 2023–24, with 15 goals in 34 league matches) and then played 33 Ligue 1 matches in 2024–25 with 4 goals.
In September 2025 he moved to Germany's 2. Bundesliga with 1. FC Magdeburg before returning to Japan with Urawa Red Diamonds on a free in February 2026. The journey across France's lower and top divisions has made him one of Japan's most well-travelled centre forwards.
A powerful, physical centre forward whose acceleration and aerial presence translate well to the international game, Onaiwu is best known to Japan fans for his hat-trick (3 goals in 2 matches) during 2022 FIFA World Cup qualifying — a remarkable strike rate that put him firmly on Hajime Moriyasu's radar.