Hassek descended from a musical family. At the age of 14 he attended the Potansky music school in Vienna and thereafter studied composition at the Vienna Academy for Music under Joseph Marx, Otto Siegl and Rudolf Kattnigg as well as piano under Eugenie Wild-Volek and Ludwig Czaczkes. From 1944 until 1954 Theo Hassek performed as pianist and interpreter of his own works. His pianistic repertoire ranged from Johann Sebastian Bach to the 20th century and comprised also technically demanding works such as Ravels "Gaspard de la Nuit", "Miroirs", Balakirews "Islamey" and Liszts h-moll-Sonata. 1950 he passed the exam for piano and 1952 got his matriculation exam for composition theory. After three years as head of the music school in Kittsee (1951 to 1953) he decided for an "ordinary" occupation and worked as agent for a publishing house, as warehouse keeper and for property management. Beside he composed several music works amongst others for the subsequent composer and clarinet of the Vienna Phlharmonic Orchestra Arnold Hartl. 1957 he married Ingeborg Smolik. After 1962 he turned back towards music an actively played as member of the three-man band "The Gamblers" together with his cousin Erhard Hassek beat and dancing music. After a number of engagements in shady bars they succeeded a breakthrough at the winter Olympic games at Innsbruck 1964 where they performend at the Sporthotel in Axamer Lizum. From now on the band was assigned at Intercont Hotels in Genf, Frankfurt, Hannover, Düsseldorf and Vienna. After the birth of his son Helmut (born 1967) the family moved back to Vienna in 1971. From 1971 Hassek turned also to traditional Vienna music "Wienerlied" and performed at Heurigen Christ 42 in Vienna Stammersdorf and at Bach-Hengl in Grinzing, where he was part of the band "Die Charmanten Wiener". Apart from a few performances at four-hand piano (together with his longtime partner Helmut Hofmann) he terminated his career of an active musician due to health reasons in 1983 but henceforth worked as an editor and arranger for vor Vienna based publishers (i. a. Weltmusik Hochmuth-Verlag, Hermann Schneider). In those days he arranged over 500 Werke (predominantly easy listening) for piano. He retired 1988 but kept composing. Theodor Hassek died from heart disease on 28 October 2009.
Theodor Hassek composed over 100 works of various genres, from neoclassical piano and chamber music works, piano works of higher level, pop music, traditional Vienna music "Wienerlied" to spiritual music (mass). His legislating is characterized by the principlet that music should be pleasure for the listener. Problematic music won't be found among his works. Nevertheless, all works are distinguished by inventive ideas, diligent elaboration, exhilarant shortness and the absence of importunate redundancy. Although his compositions have an unmistakable typical piano sound they are predominantly committed to the form rather than the sound. Hence transcriptions of earlier works written for other instruments can be found in the chamber music as well as in compositions for two and four handed piano.
In his last years he devoted the composing of spiritual choral works – stimulated by Josef Böck – whose performance he missed. The originals of his notes, manual sketches and some recordings are in possession of the family and will be given to the "Musiksammlung der Wienbibliothek im Rathaus" in Vienna at a later moment.