Pinnock began his career in non-League football with Dulwich Hamlet and Forest Green Rovers, before rising to prominence in the EFL.

He began his career as a fullback for Dulwich Hamlet, leaving the Millwall academy aged 15. He had a short loan spell with Holmesdale, returning to Hamlet to become a centre-back. He was part of the side that won the Isthmian League Division One South in 2013, winning the Player of the Year Award in back-to-back seasons (2014/15 and 2015/16).

In June 2016, he joined National League side Forest Green Rovers on a two-year contract. After 45 appearances and three goals in the 2016/17 season, he helped Rovers to promotion to the EFL for the first time in club history.

In June 2017, Ethan signed a three-year deal at Barnsley in the Sky Bet Championship. He finished his first injury-affected season with only 15 appearances as The Tykes were relegated to League One.

He missed just two games the following season as Barnsley gained automatic promotion back to the second tier. He was named the club's Player of the Year and was named in both the PFA and EFL League One Team of the Year.

In July 2019, he joined Brentford. He made 40 appearances and scored twice during a season that took The Bees to the Play-Off Final at Wembley Stadium. After a 2-1 loss to Fulham, the following season they went one better, beating Swansea 2-0 to confirm their first promotion to the top-flight since 1935. He was named in the 2020-21 PFA Team of the Year.

That season also saw a first call-up and senior cap for Jamaica. His first match came against the United States, earning him a place on the provisional squad for the 2021 Gold Cup.

His Premier League debut was on 13 August 2021, when Brentford beat Arsenal 2-0 on the opening day of the 21/22 campaign. He would go on to play 35 games in all competitions, scoring his only goal against Liverpool during a 3-3 draw at Brentford Community Stadium.