In 1945 he was transferred to the newly-formed Ministry of Public Security of Poland (MBP). In time he as promoted to colonel; he served in various offices and departments, eventually in 1951 ending up in the 10th Department, where he was one of the leading officers. Department 10 dealt with the members of the party, from the political bureau – the highest authority – to the lowest cells. He received orders personally from First Secretary of the Polish United Workers' Party Bolesław Bierut, and arrested such notable people as politician Władysław Gomułka, General Michał Rola-Żymierski and Cardinal Stefan Wyszyński. He had access - sometimes unique - to many secret documents. He had carried out an interrogation of Noel Field on 27 August 1949 in Budapest 44 and was involved in the arrest and subsequent interrogation of Noel's brother, Herman Field, a United States citizen, who went to Warsaw in 1949 to look for Noel but was secretly detained at the airport and kept in a 10th Department prison for five years, until Światło broke the story in his radio broadcasts. In his work, Światło, like many other communist secret police agents, used torture and forgery.
In November 1953, First Secretary of the Polish United Workers' Party Bolesław Bierut asked Politburo member Jakub Berman to send MBP Lieutenant Colonel Józef Światło on an important mission to East Berlin. Światło, deputy head of MBP Department X, together with Colonel Anatol Fejgin, were asked to consult with Stasi chief Erich Mielke about eliminating Wanda Brońska. Światło, however, after the death of Stalin in March 1953 and arrest of Lavrentiy Beria in June that year, became afraid for his own life.
The two officers traveled to Berlin and spoke with Mielke. On 5 December 1953, the day after meeting the Stasi chief, Światło defected to the U.S. military mission in West Berlin. He left family - wife and two children - in Poland. The next day, American military authorities transported Światło to Frankfurt and by Christmas Światło had been flown to Washington, D.C., where he underwent an extensive debriefing. It has been reported that US intelligence put together some fifty lengthy reports from Swiatlo's interrogations.
Światło's defection was revealed in Poland by the Polish Press Agency on 25 October, with Światło labelled a traitor and provocateur. It was, however, widely publicized in the United States and Europe by the American authorities, as well as in Poland via Radio Free Europe, embarrassing the Communist authorities in Warsaw - the first international press conference with Światło took place on 28 September. Światło had intimate knowledge of the internal politics of the Polish government, especially the activities of the various secret services. Over the course of the following months, American newspapers and Radio Free Europe (in the \"Behind the scenes of the secret service and the party\" cycle) reported extensively on political repression in Poland based on Światło revelations.
Capitalizing on them, in what was known as "Operation Spotlight", RFE saturated the airwaves with some 140 broadcasts by Światło as well as 30 special programs. Światło's RFE broadcasts were not merely serialized but subsequently air dropped by special balloons over Poland. Światło detailed the torture of prisoners under interrogation and politically motivated executions and struggles inside the Polish United Workers' Party. Among other activities, Światło had been ordered to falsify evidence that was used to incriminate communist politician Władysław Gomułka, who would become first secretary of the Polish United Worker's Party in 1956, and personally arrested him. He had also arrested and created evidence against Marian Spychalski, the future Minister of National Defense, who was at the time a leading politician and high ranking military officer. All the intelligence, counterintelligence, and public security institutions of Peoples’ Poland were compromised.
Światło wrote a memoir about his life.