During World War I, German occupiers were slightly more liberal in their ways than their previous Russian counterparts, and allowed the official registration of sports clubs on Polish territory.

In the late 1930s, Polonia became one of the powerhouses of Polish football, with players such as: Jerzy Bulanow, Władysław Szczepaniak, Erwin Nyc, and Henryk Jaźnicki capping for the national team.

[8] KSP achieved many successes during the two decades between the world wars, establishing itself as a club with sections in: hockey, athletics, football, fencing, tennis, volleyball, basketball, swimming, hazena, cycling, and boxing.

The rivalry between these two clubs continued to grow in the interwar period, during which they played against each other a total of 27 matches; Polonia secured 14 victories, Legia won 8 times, whereas 5 games ended in a draw.

Authorities of the German occupying forces thus forbid Poles from practicing sports; despite this, KSP continued to play illegally, taking part in various underground initiatives and winning the secret Varsovian Championship in 1942 and 1943.

[10] Pre-war footballers of Polonia, Adam Kogut and Julian Niemczycki, and athletes Wincenty Fryszczyn and Wacław Znajdowski, were among Poles murdered by the Russians in the large Katyn massacre in April–May 1940.

[12] During the Stalinist period, Polonia's name and colours were changed; every Polish football club received a national sponsor, such as the army, militsiya, mining industry, etc.

[16] One of the reasons behind this was that all the young men, promising footballers to be from all over Poland, and especially the Warsaw youth academies, were called up for compulsory army training, which in the PRL lasted about 5 years or sometimes even longer.

In 2009/2010, after several changes in managerial positions, and with the team performing below expectations (15th place – mid-season), the club chairman Józef Wojciechowski decided to employ the former FC Barcelona captain José Mari Bakero as manager.

[30] Although the team was last in the league at that point, Bakero managed to save the season by avoiding relegation and winning against the local rivals Legia Warszawa for the first time in 10 years.

[30] The former Polish national team manager Paweł Janas, already working at the club as football director, was installed as a replacement for the Spaniard, signing a 2-year contract.

On 6 June 2011, Polonia's star playmaker Adrian Mierzejewski was sold to Trabzonspor for a record fee in Polish football history of 21 million zł (5,3 mln euro).

[36] On 11 October 2011, Polish football legend Włodzimierz Lubański was appointed as vice-chairman/sports director to relieve Józef Wojciechowski of his duties, in order to secure Polonia's immediate future.

[34] Mid-season however, with Polonia surprisingly sitting in 3rd place in Ekstraklasa, merit to young coach Piotr Stokowiec, the worst possible scenario turned out to be true as Król was revealed as a financially unreliable owner.

Delays in contract payments caused a repeat of the turmoil from the summer; in 2013 Polonia lost more than half of its first team again, and nearly all the high-priced players left on free transfers.

Almost the entire defensive block was destroyed with Đorđe Čotra, Adam Kokoszka, and the experienced Marcin Baszczyński leaving the club; the recently signed midfielder Hołota escaped Król's mismanagement too.

[45] Among last-minute deals and loans of young talented players, Polonia surprisingly signed the experienced 23-year-old Estonian national defender Igor Morozov from Levadia Tallinn.

[48] Despite efforts from the association of Polonia fans to regain control over the club to clear the debts with new sponsors and ensure KSP's survival, Król was not interested.

Unfortunately the game had to be suspended after 36 minutes, due to third party hooligans as fans of Legia Warsaw (supposedly supporting KS Łomianki) put up illegal banners, threw objects onto the playing field, and finally invaded the pitch in numbers, resulting in riots with the police and subsequently ending the match for both clubs.

Polonia Warszawa S.A. at their first press conference also made a strong intent of building a modern General Kazimierz Sosnkowski Municipal Stadium within five years (until 2020), with the goal of returning to the Ekstraklasa in the meantime.

Wojciech Szymanek remained as coach, and on the 30 of April Piotr Kosiorowski was named as the sporting director, along with the presentation of ambitious plans for the future to reach the Ekstraklasa by 2029.

[73] Due to Polonia's recognisable brand and long history, this move was seen as an important event in Polish women's football in general; Jacek Sękowski from Piastovia Piastów became the team's first coach.

[76] They continued their success in the league games, winning 6:0 against the reserves of AZS Uniwersytet Warszawski in their inauguration match of the IV liga mazowiecka and followed it up with another high victory of 11:3 against Świt Barcząca.

The club and its ultras produced unique memorabilia for the event whereas the Poczta Polska introduced a special postage stamp for the occasion, which was designed by Agnieszka Sancewicz on fluorescent paper and printed in a circulation of 135,000.

[79] Thanks to fan initiatives: a new song was recorded; the Palace of Culture and Science, the National Stadium, and the Śląsko-Dąbrowski Bridge were illuminated in KSP colours; and messages were displayed on screens throughout Warsaw's public transport to remind the capital of its oldest existing club's history.

[80] Polonia triumphed 2:0 over Wissa Szczuczyn on the main day of the celebrations,[81] ending the autumn round of games in a strong second position of the league table and leaving the fans in an ecstatic mood on the 110th birthday of the Czarne koszule.

[84] The victorious sold-out match between then-group-leader Legionovia Legionowo that finally put KSP in the lead attracted 5000 fans to the home stadium, and many online viewers, who took part in and witnessed a display of Ekstraklasa-level support.

The friendship between Polonia and KS Cracovia used to be the oldest in Poland (starting in the 1930s); though officially it was ended in 2017 due to disagreements between ultras, friendly relations between the clubs and their supporters persist.

Famous supporters include Doman Nowakowski, Maciej Dowbor, Jan Englert, Michał Listkiewicz, Stanisław Tym,[90] Kazimierz Górski, Krzysztof Ibisz, Marek Jurek, Grzegorz Jankowski, Wojciech Wysocki, Hanna Śleszyńska, Henryk Chmielewski, and Tomasz Konatkowski.

[91] Adam Bahdaj wrote a famous book entitled Do przerwy 0:1 (0:1 at half-time) which centres around local children playing football inspired by Polonia's team.