Korboński graduated on 4 September 1925. Instead of training to become a barrister, in 1927 he decided on a two year supervised internship program at the Office of Attorney General (first in Poznań, and then in Warsaw) which exempted him from the obligation to take the bar examination. In that same year, in compliance with the regulation on higher officer ranks to war veterans issued by the minister of military affairs, he reported to the Cadet Officer School in Śrem.
Upon the completion of a six week training course (in which he ranked eighth in his class), he was promoted to the rank of a Second Lieutenant, and assigned to the 57th Infantry Regiment (former 1st Regiment of Wielkopolska Riflemen) in Poznań. Later on, during the war, he was promoted to the rank of a Lieutenant (under Order No. 79 of the Commander-in-Chief of the Home Army [Armia Krajowa, AK], Brigadier General Stefan Rowecki, known by his pseudonym “Grot”, of the 20 April 1943), and then, after a year, to the rank of a Reserve Captain of the Polish Army (under Order No. 487, issued by the Commander-in-Chief of the Home Army, Division General Tadeusz Komorowski). Moreover, he held the rank of Major, awarded to him by Peasants’ Battalions (Bataliony Chłopskie, BCh). The death of his mother, who passed away on 25 September 1927, at the age of 49 as a result of an unsuccessful gall bladder operation, came as a shock to Korboński. His mother was buried in the family grave in Słupca. Her coffin was placed next to that with the mortal remains of her daughter Stefania, who died young at the age of 17. The other two sisters of Korboński, Jadwiga and Maria, died around the same time: the first in Radziejów Kujawski, Poland, in 1990 and the other one in California, a year later. As for his brother, he was killed by a V2 rocket in London during World War Two. Stefan Korboński financially supported his sister Jadwiga and kept in touch with his relatives until the end of his life. In 1929, Korboński refused a partnership offer by Jan and Stanisław Sławski, the owners of a renowned law office in Poznań. Acting on the advice of his cousin, Zygmunt Graliński, he moved to Warsaw. They founded a law partnership mutual in nature, with equal rights for both partners.
The underlying reason for such a decision was his interest in politics (Graliński was a Deputy acting on behalf of the Polish Pasant Party “Wyzwolenie” [Polskie Stronnictwo Ludowe “Wyzwolenie”, PSL-Wyzwolenie]), which he showed as early as his school days. Korboński thought that it was the peasantry who were the social group with the biggest political potential, causing him to join PSL-Wyzwolenie upon graduation from university.
In 1926, during the May Coup d’État, he supported Józef Piłsudski, just like the aforementioned Party. After moving to Warsaw, Korboński stayed at the Hotel Sejmowy on Wiejska St. There, in 1930, after being entered on the Warsaw list of attorneys at law, he opened (at that time informally) his law office, which was then moved to No. 23 building on Wilcza St. Soon Korboński was appointed to represent the Poznań Bar at the Supreme Court in Warsaw, which, undoubtedly, strengthened his position in the capital. At the beginning of the 1930s, the Graliński & Korboński Law Office, thanks to Szymon Landau, with whom both partners had been acquainted, was appointed to act as legal counsel to the Warsaw Regional Power Plant in Pruszków. Moreover, the firm was appointed to provide legal services to the Overseas Bank from London and to Prudential Insurance Company, a global insurance business affiliated with the bank, as well as to the Polish affiliate of the latter, the “Przezorność” Insurance Society. It was Stefan Korboński who made arrangements for the building contracts respecting the first “skyscraper” in Warsaw (currently: the Hotel Warszawa building) when the “Przezorność” Insurance Society embarked upon its investment project connected with the construction of its new headquarters in Napoleon Square.
In the period 1934–1939 the range of Graliński & Korboński Law Office customers expanded as other insurance companies (Poznańsko-Warszawskie Towarzystwo Ubezpieczeń and Vesta) joined in, followed by Wspólnota Interesów, a great mining and industrial business group from Upper Silesia andDąbrowa Coalfield, and its affiliated company Robur. The above businesses were managed by one of the wealthiest Polish industrialists, Alfred Falter, whom Korboński met after the end of World War Two in New York. It was Falter to whom the Graliński & Korboński Law Office was indebted to for the most important transaction they had ever handled: the sale of the Rybnik Coal Mining Company (Rybnickie Gwarectwo Węglowe) by Baroness Rothschild from Paris, at 10% of its value, to benefit the Polish Treasury. The execution of that transaction was prevented by the outbreak of World War Two. As a result the advance payment in the amount of 50,000 zlotys made by the Baroness in attorney’s fees was lost as well. Two years before the outbreak of the war Korboński was appointed legal advisor to Bank Handlowy. Its Supervisory Board was chaired by a former minister for foreign affairs of the Republic of Poland, August Zaleski (in the years 1947–1972 he was to become the President of the Republic of Poland in Exile). American Great War veteran and multimillionaire Charles MacDaniel, conducting, together with Prince Eustachy Sapieha, the logging operations in Różański Forest, empowered Korboński to act on his behalf within the scope of his business done in Poland.
The Graliński & Korboński Law Office became a popular meeting place at the turn of 1929 for the most outstanding representatives of the Warsaw Bar, including Leon Berenson, Eugeniusz Śmiarowski, Wacław Szumański.
Thanks to Graliński, Korboński met quite a number of renowned Polish politicians, including former Deputy Prime Minister Stanisław Thugutt and his son Mieczysław with whom he became friends and later on, during the war, closely cooperated. Henryk Kołodziejski, Stanisław Stempowski, Mieczysław Niedziałkowski, Norbert Barlicki, Stanisław Dubois, Stanisław Garlicki, Ludwik Cohn, Adam Obarski, Andrzej Czajkowski, Jan M. Borski, as well as Wincenty Witos, a three time Prime Minister of the Republic of Poland, and Maciej Rataj, the Speaker of the Polish parliament at the time of the May Coup d’État, were also among Korboński’s friends. On 29 June1930, Korboński, acting in his capacity as assist
ant to Kazimierz Bagiński, took part in the congress of Centrolew (a coalition of several Polish leftist and centrally-oriented political parties) held in Cracow at which representatives of a number of Polish workers’ and peasants’ political formations opposing the government under Marshal Piłsudski met, including the Polish Socialist Party (Polska Partia Socjalistyczna, PPS), the National Workers Party (Narodowa Partia Robotnicza), the Christian Democratic Movement (Chrześcijańska Demokracja), PSL-Wyzwolenie and PSL-Piast, and Peasants Party (Stronnictwo Chłopskie). Bagiński and Korboński called upon 2,000 peasants from Miechów area to participate in the meeting.
Since the congress resulted in the arrest of 19 opposition deputies on the night of 9 September 1930, the Graliński & Korboński Law Office made arrangements to represent them in court. Afew months later Korboński’s name appeared in newspapers for the first time. He had his letter protesting the removal of Kazimierz Bagiński’s name from the list of election candidates published in “Robotnik” on 7 November 1930. On 27 November 1930, after Bagiński’s release from prison (in Grójec, where he was finally sent from Brześć), the Graliński & Korboński Law Office started preparing to act before the court as the defence counsel of the accused in the Brześć Trials. At the turn of September 1933, Korboński accompanied Bagiński during his
journey to Zakopane when the latter, in order to avoid the imprisonment to which he had been sentenced, voluntarily went into exile to Czechoslovakia. Beginning in March 1931, Bagiński persuaded Korboński to intensify his involvement in Peasant Party (Stronnictwo Ludowe, SL) activities, for example participating in folk celebrations in Biała Podlaska and Lublin. Maciej Rataj, the SL leader after the emigration of Witos and Bagiński, made him accept the responsibilities of the chairman of a local SL unit in Łomża in 1934.
Then, in the 1936 election to the governing bodies of SL on the level of a voivodeship (a province), Korboński was unanimously elected chairman of the Białystok Provincial unit of SL.
He was in office until the outbreak of World War Two. In the middle of September 1937, Stefan Korboński and Stanisław Dubois, together with Kazimierz Bagiński, then in exile, participated in the funeral ceremony of Tomasz Masaryk, the first President of Czechoslovakia. At a mass meeting during the folk celebrations held at a former Camaldolese monastery on Wigry Lake near Suwałki in 1937, Korboński proclaimed a farmer strike lasting from August 15 to 25, one of the many then sweeping the country. As a result of the strike wave, 42 members of the peasant movement lost their lives. They were posthumously honoured at the SL congress held in Cracow on 27–28 February 1938. In Warsaw on 10 July 1938, Korboński married Zofia Ristau, the daughter of a chemical engineer, Wacław Ristau.
The newly wedded couple went to Paris on their honeymoon, a mention of which was made in Wincenty Witos’ memoirs. Korboński met Witos in person at an SL meeting in Białystok after he returned from exile in Czechoslovakia in the middle of March 1939. The next opportunity for their meeting presented itself at the place of the Graliński family, soon before the outbreak of World War Two.