Frinovsky was one of the major beneficiaries of the first purge of the NKVD that followed dismissal of its head, Genrikh Yagoda. He had had some kind of falling out with Yagoda, but was on good terms with Yagoda's successor Nikolai Yezhov. On 16 October 1936, Frinovsky was appointed Deputy Chairman of the NKVD, which made him third in seniority within the Soviet security apparatus.
Frinovsky's position was significantly enhanced following a period of turmoil within the NKVD, which led to Yagoda's dismissal. He had previously had a falling out with Yagoda but had since restored relations with his successor, Yezhov, and soon became the third-most powerful figure in the Soviet security apparatus.