Onuachu moved to Danish club FC Midtjylland in 2012, on a scholarship, from their affiliate team in Nigeria, Ebedei. He was a prolific scorer for their youth team, and made his first-team debut in the Cup later that year, before making his league debut in December 2012. In June 2013 he signed a new three-year contract with the club, before extending it for a further three-years in August 2015. In early 2015 he was loaned to Vejle BK, before returning to FC Midtjylland ahead of the 2015–16 season.
In August 2019 he signed for Belgian club Genk.
In February 2015 he was called up to the Nigerian under-23 national team.
In March 2019 he received his first call-up to the Nigerian senior national team.
On 26 March 2019, Onuachu scored his first goal for the Nigerian team in a friendly match against Egypt. The goal was scored within the first ten seconds of the game, and the fastest ever scored for Nigeria. Following the goal he became "toast of Nigerian football" with "his coach, team-mates, journalists and fans talking about him". He was selected to the Nigeria squad for the 2019 African Cup of Nations. He played in Nigeria's 1-0 win over Burundi.