On April 19, 2005, Kobbie Mainoo was born in Stockport, England. He goes by Kobbie Boateng Mainoo in full. He is of mixed heritage and possesses British nationality. In 2023, he celebrated his 18th birthday.

His parents, Fepx Mainoo and Abena Herold, witnessed his first professional contract signing with Manchester United and have been supportive of his sporting career. But on September 29, 2018, his mother Abena Herold wedded Rob Herold, ending their previous marriage. RNH Management Services Ltd.’s project manager is Rob Herold.

Ama and Efia Mainoo were raised together. Ama studied sports business management at Leeds Beckett University and graduated in 2019, while Efia completed her undergraduate studies in biochemistry at the University of Birmingham in 2018.

Mainoo began his junior career at Cheadle & Gatley Junior Football Club before joining Manchester United at the age of nine. He was raised in Stockport, Greater Manchester.

Kobbie Mainoo signed his first professional deal with Manchester United in May 2022. His efforts, which included a strong midfield showing in the EFL Trophy match against Carpsle United, led to a call-up to the senior squad for training in October 2022.

After making great strides, on October 16, 2022, he was placed on the bench for the first time in order to be ready for a Premier League game against Newcastle United. On January 10, 2023, Mainoo made his competitive debut as a starter in a 3-0 EFL Cup victory against Charlton Athletic. Two days later, on February 19, he made his league debut by substituting in a 3-0 success over Leicester City.

Mainoo then accompanied Manchester United to the United States for their 2023–24 preseason tour. Sadly, he was sidelined by an injury at the beginning of the next season. On November 26, 2023, Mainoo celebrated his comeback from injury by starting in a competitive match for Manchester United, helping the team defeat Everton 3-0 in the Premier League. Mainoo has represented England at the under-17, under-18, and under-19 levels internationally. He also possesses epgibipty to represent Ghana.