His first theatre ensemble – still partially an amateur one – was a summer theatre performing at the Promenada garden located on the outskirts of Warsaw of that time. The theatre’s repertoire was incredibly wide-ranging: beginning with Juliusz Słowacki's Mazepa and ending with Charley’s Aunt, a popular farce by Brandon Thomas. The theatre used to produce a premiere a week, as a result of which, to quote Sempoliński himself, “the acting technique was learned quickly, but superficially”. At the same time, Sempoliński joined the choir of a well-recognised Nowości Operetta Theatre located at Daniłowiczowska Street. There, the would-be brilliant artist had the chance to meet the greatest stars of the Warsaw Operetta, which is said to have been a decisive factor in leading his artistic career towards the field of the Muse of comedy.
The first professional theatre in Sempoliński’s career was Sfinks, an artistic cabaret well-known in Warsaw. He made his debut on Sfinks’s stage on July 9, 1918 with a song entitled Lecą mareczki, lecą, lecą – a satire on a rising hyperinflation. As it was unseemly for a student of the school of economics to perform under his own name, for a short time he used a pseudonym of Bohdan Kierski. Aside the entire pantheon of then popular, and today partly or completely forgotten actors, his stage colleagues were the greatest theatre legends of the inter-war period. Among them: Hanka Ordonówna, on the threshold of her career as a singer, and a future remarkable actor Michał Znicz. Sempoliński spent two seasons at Sfinks. It is said that he left the theatre due to thwarted ambitions. His friend Znicz received an invitation to join a newly established Qui Pro Quo Theatre. Next, Sempoliński performed at Nowości, still managed by its legendary director Józef Śliwiński. There, he developed his talent as an actor and operetta singer.
This valuable experience, as well as a good knowledge on the fin de siècle's style paid off in the subsequent years and helped him in creating the most remarkable stage characters. In 1921 Sempoliński joined the ensemble of the Stołeczne Theatres managed by Ludwik Heller. However, he still maintained good relations with Nowości. Those days he also made his first attempts as a director, which were crowned with an examination passed before the board of ZASP, the Association of Polish Stage Artists. From 1923 to 1927 the artist gave performances mostly outside Warsaw: from 1923 to 1924 at Nowości – the Kraków-based equivalent of the renowned Warsaw Operetta, and between 1924 and 1926 at the Na Pohulance Theatre in Vilnius. He returned to Warsaw in 1927, and was an actor of the Niewiarowska Theatre, managed by a great singer and a star of the Warsaw Operetta Kazimiera Niewiarowska. His career at this theatre was interrupted by Niewiarowska’s tragic death.
The most fruitful artistic period in Sempoliński’s career started in 1928 when he was employed by Andrzej Włast, the director of the legendary Morskie Oko Revue Theatre in Warsaw. The artist performed there for three seasons until 1931. During that time, his famous song Tomasz, ach Tomasz, ach powiedz, skąd ty to masz? was created. The artist performed the song so brilliantly for the next few decades that its refrain eventually became a popular saying. From 1932 to 1935 Sempoliński was a member of the revue theatre company called Wielka Rewia, which continued the artistic vision of the Morskie Oko Revue Theatre. It was in this revue theatre located on 18 Karowa Street that Sempoliński surprised the audience with a tribute to the style popular at the turn of the 19th and beginning of the 20th century. At that time, this epoch was mocked upon as it was considered the ultimate in bad taste (the 1930s were the beginning of modernity with all its virtues and vices).
At that time Sempoliński sang his famous song Warsaw 1900, and created the unforgettable character of the Master of Ceremonies at carnival balls. Most probably, it was the first remembrance of that style brought back to the epoch. The last theatre company Sempoliński performed at was Ali-Baba, established by Andrzej Włast and Kazimierz Krukowski a few months before the outbreak of the Second World War. It was located at the same Panorama building, however at a small stage, which had formerly belonged to the so-called Malicka Theatre. There, Sempoliński created two great stage characters. The first one was the renowned Ostatni Posłaniec The Last Messenger), a song written by Władysław Szlengel and Tadeusz Wittlin as a nostalgic good-bye to the disappearing atmosphere of old-time Warsaw, performed at an opening show entitled Sezonie Otwórz Się (Open Season) on April 5, 1939. This is how Sempoliński recalls the moment of the song’s creation:
On my everyday walk to the theatre I used to pass by the Europejski Hotel where I noticed an old man, a messenger, who was left with only a red hat out of the entire uniform and whose job was to distribute advertising leaflets in return for a poor pay. His miserable look inspired me to recreate such a character on stage. I shared my idea with Włast, who instantly liked it and wrote the song’s lyrics himself. However, he considered the lyrics a failure and did not even let me read them. Two of the authors he asked for help wrote lyrics independently, which were then put together to eventually create a beautiful and touching song. Music based on the old motifs of the Warsaw waltzes was composed by Sygietyński. In effect, one more hit was created, which I added to my repertoire.
I invited the messenger for the dress rehearsal and premiere. Thanks to the make-up, I was deceptively similar to him. I experienced the effects of my efforts as early as at the premiere. The moment I appeared before the curtain during a prelude, an elder man, looking alike Wojciech Kossak, stood up and said in a loud whisper so that the whole room could hear:
It’s my messenger. It turned out that by chance I picked up for my model a messenger who forty years earlier, when this guy used to arrive in Warsaw for his revels, had been employed by him as a messenger. If he interrupted my performance only during the premiere, I would forgive him his enthusiasm. Yet, he would persistently come to our show, every time accompanied by different friends, whom he used to inform about my stage character (loudly enough for the whole room to hear).
The second remarkable performance created at the end of May 1939, in the atmosphere of increasing uncertainty after the Nazi Germany had renounced a non-aggression pact, was Titina. Dressed up as Adolf Hitler, Sempoliński sang a song composed by Charlie Chaplin with the Polish lyrics Ten Wąsik (This Moustache) written by Marian Hemar. The artist recalls the highly dramatic circumstances surrounding this performance and the heated reaction of the audience:
A month following the opening night of the revue, the German Ambassador protested at our Ministry of Foreign Affairs against making fun of Hitler. We were bewildered as this stage character was one of the highlights of our show. In my attempts to rescue the show, I shaved the moustache, but it did not help much. The observers from the German Embassy were watching whether the order was obeyed. I was forced to dress up as Goebbels. They allowed it.
Luckily, the word Hitler was not mentioned in the song Ten Wąsik (This Moustache). Therefore, they had no grounds to pick on it and the song remained in the show’s programme. The character of Hitler caused such a great sensation that even Jews wearing gabardines, who did not understand much in Polish, used to pack the audience. For them, the character of Hitler was the main attraction. When I was standing on a pedestal as a wax figure of the Führer, I felt excitement in that part of audience and I heard the whisper: Hitler... Hitler...The then touts and claqueurs, Morycy and Rudy, earned quite a sum from re-selling tickets. Who would have thought that these productions would soon bring me so much trouble.
Photographs featuring Sempoliński as Hitler welcomed the German soldiers entering Warsaw destroyed in October 1939. Sempoliński managed to escape to the Soviet side, which saved his life. He was sought after by the German police charged with making fun of the Führer and a death sentence.
From 1940 to 1941 Sempoliński used to perform in Vilnius at the Miniatury and Lutnia Theatres. As a result of the subsequent German occupation of this territory, he used to hide until 1944 under the pseudonym of Józef Kalina. His various jobs included that of a herdsman. Between 1944 and 1945 the artist lived in Vilnius again and worked at the revived Lutina Theatre. In 1945 Sempoliński moved to Łódź where he performed for a short time at the Kameralny and Bagatela Theatres, while beginning with 1946 he joined the ensemble of the Syrena Theatre. The artist remained faithful to this theatre till the end of his professional career (excluding short breaks for performances given at the Nowy Theatre in Warsaw from 1952 to 1953 and in Łódź: 1949-1952). Sempoliński also lived in Kraków for a short period of time when in 1946 Marian Eile offered him a contract at the renowned 7 Kotów Cabaret.