Between 1910 and 1913, Pimmer was a member of Slavia Prague, the country's leading team, helping his side win back-to-back Charity Cup titles, beating AFK Kolín 4–1 in the 1911 final[2] and beating FC Viktoria 4–3 in the 1912 final on 26 May.[3] On 17 September, Pimmer was one of the eleven footballers who played in Slavia's first-ever match in the newly-founded Bohemian championship, keeping a clean-sheet in a 3–0 win over Olympia.[3] In the following year, in 1913, Pimmer helped Slavia win its first championship title, doing so with seven wins and one defeat.[4]
On 28 October, Pimmer played for the representative team of ČSF in a friendly matches against England amateurs, which ended in a 2–1 loss.[5] In May 1911, Pimmer participated in the 1911 UIAFA European Football Tournament at Roubaix, an unofficial European Championship organized by UIAFA, as a member of the Bohemian team (ČSF), which was made up largely of players from Slavia and coached by Johnny Madden.[2] In the build-up for the tournament, Pimmer helped Bohemia/Slavia win a few friendly matches against British clubs, such as Aberdeen (3–2), Ipswich Town (4–0), and Eastbourne Town (5–4).[2] In the tournament, Pimmer helped Bohemia beat Belgium 6–1 on 25 May; USFSA's France 4–1 in the semifinals on 28 May, and then AFA's England 2–1 in the final on 29 May,[2] saving a 83rd-minute penalty kick after Coach Madden shouted to him "BE CALM!"; thus contributing decisively in Bohemia's triumphant campaign. The French newspaper L'Auto spelled his name as "Plunner Karol".[6]