Brennan joined the Forest Academy when he was only eight years old, making his professional debut against West Bromwich Albion in August 2019.

He spent the 2020-21 campaign on loan in League One with Lincoln City, scoring 13 goals in a total of 48 appearances for The Imps.

Since returning to The City Ground, he has established himself as an important player and picked up the EFL Young Player of the Month award in September.

Johnson has earned six caps for Wales having made his senior international debut in November 2020.