In July 2014, after more than a decade with Sturm Graz, Graz native Kainz transferred to Rapid Wien for an undisclosed fee.[5] He signed for Rapid until summer 2017.[6]
In June 2016, Kainz joined Werder Bremen on a four-year contract for a reported €3.5 million transfer fee.[7] On 18 March 2017, he scored his first goal for the club in a 3–0 win against Leipzig.[citation needed] Having been introduced as a substitute in the 88th minute he found the net two minutes later in a match that saw all three of Werder Bremen's Austrian players score in the club's highest win of the season.[8][9] Two matches later he assisted the last goal in Werder Bremen's 5–2 away victory against Freiburg via free kick, after being brought onto the pitch in the 76th minute.[10]
In the first match of the 2018–19 season, a 6–1 win against Wormatia Worms in the first round of the DFB-Pokal, Kainz assisted the first goal and scored the second in a "Man of the Match" performance.[11][12]
In January 2019, Kainz joined 1. FC Köln on a three-and-a-half-year contract. The transfer fee paid to Werder Bremen was reported as €3 million.[13][14] In March 2022, he signed a new contract until 2024.[15] In January 2023, he extended his contract until 2025.[16] He became club's captain ahead of the 2023–24 season, following Jonas Hector's retirement.[17]
^Kleiner, Stefan; Knöbl, Ralf (2015) [First published 1962]. Das Aussprachewörterbuch [The Pronunciation Dictionary] (in German) (7th ed.). Berlin: Dudenverlag. pp. 373, 492. ISBN978-3-411-04067-4.
^Krech, Eva-Maria; Stock, Eberhard; Hirschfeld, Ursula; Anders, Lutz Christian (2009). Deutsches Aussprachewörterbuch [German Pronunciation Dictionary] (in German). Berlin: Walter de Gruyter. pp. 513, 632. ISBN978-3-11-018202-6.
^"Neuzugang: Florian Kainz" [New signing: Florian Kainz]. SK Rapid Wien (in German). 22 July 2014. Archived from the original on 2 February 2015. Retrieved 21 February 2015.