Charlton Athletic's Nigeria Under-20 international Semi Ajayi played for a Norwich City XI against St Neots Town on 20th July 2013, which City won 3-1.

Semi had been with Charlton since the age of 9 and had spent December 2012 on loan at non-league Dartford.

Despite being offered a contract by Norwich, Ajayi joined Arsenal on trial and joined their Academy on 23rd September 2013. Ajayi scored on his debut for the under-21s in a 3-0 victory over Blackburn Rovers. He trialled with Ipswich during the 2014/15 season.

The 21-year-old centre-back joined the Championship club on loan in March and Cardiff boss Russell Slade made the move permanent on 19th June 2015, with Ajayi agreeing a two-year deal, subject to international clearance.

Despite being capped by Nigeria at Under-21 level, he could not break into the Cardiff first team and was loaned to AFC Wimbledon on 29th September 2015 for a month, He made five appearances.

On 23rd November 2015, he was loaned to Crewe and made 13 appearances.

Rotherham signed Ajayi on loan from Cardiff on 30th January 2017 until the end of the 2016/17 season. He played on 1st April 2017 when a 1-0 home defeat to Fulham saw the Millers relegated to League One.

It was agreed on 20th March 2017 that Ajayi would transfer permanently to Rotherham in the summer of 2017 on a three year deal.

In 2018, he made his international debut for Nigeria.

Ajayi and Rotherham were relegated to League One on 27th April 2019. Two days later, he was linked by Nigerian sports website Sports247.ng with a move to the Canaries. He had made 48 appearances for the Millers in 2018/19, scoring eight goals including one against Norwich.

On 20th July 2019 Ajayi signed a four year contract with West Brom who paid an undisclosed fee, Ajayi had made 108 appearances for the Millers.