Updated project metadata. Elongin is a hetero-trimeric elongation factor for RNA polymerase (Pol) II transcription that is conserved among metazoa. We solved three structures of human Elongin bound to transcribing Pol II using cryo-EM assisted by crosslinking mass spectrometry. The structures show that Elongin subunit ELOA binds the RPB2 side of Pol II and anchors the ELOB-8 ELOC subunit heterodimer. ELOA contains an N-terminal ‘latch’ that binds between the end of the RPB1 bridge helix and the funnel helices, thereby inducing a conformational change near the Pol II active center. The latch is strictly required for the elongation-stimulatory activity of Elongin, but not for its binding to Pol II, indicating that Elongin functions by allosterically influencing the conformational mobility of the active center. Structural comparisons show that Elongin binding to Pol II is incompatible with association of super elongation complex, the PAF1 complex, and RTF1, which also contains a latch element that stimulates Pol II.