He is most noted for his career in IFK Göteborg, where he was highly successful. As an international professional his track record was mixed, often just staying a year in the team. He became both Swedish and German Champion, with Göteborg and Bayern Munich respectively. Ekström became a popular, respected player in Serie A club Empoli, where he acquired the nickname 'Il Ciclone' (the cyclone) due to the exceptional speed he displayed when charging ahead with the ball in his possession, his most notable quality.
His nicknames in Sweden are "Kallebäcks-Expressen" (the express train from Kallebäck) or "Johnny Bråttom" (Johnny-in-a-hurry), again due to his speed on the football pitch. He started his youth career in the boys' team of IFK Göteborg and is one of few that made the journey all the way to the starting lineup in the first team.