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 rise to prominence occurred after a period of upheaval within the NKVD, triggered by Yagoda's removal as head. He had previously disagreed with Yagoda but had since made amends with Yezhov, who succeeded him, and eventually became the third-most senior figure in the Soviet security hierarchy.