After a year in Downtown Theatre and an other in Youth Theatre, in 1949 he got a job in National Theatre, where he stayed until 1956. His breakthrough role was Imre Madách's Moses, which he had played for 22 years, more than 700 times. On 23rd October 1956 he recited Sándor Petőfi's National Song at the Petőfi Square in Budapest in front of a crowd of 20000 people. This was the opening act of a youth protest, which protest in a few hours became a nationwide revolt against the government. Sinkovits became a member of the Hungarian Theatre and Film Association Revolutionary Committee on the 30th October. After the revolution was crushed, Sinkovits was banned from acting for a half year for his activities, and his membership in the National Theatre was terminated. Between 1958 and 1963 he worked in the Attila József Theatre. In 1963 he returned to the National Theatre and he had became one of its dominant actors for decades.