The transition between transcriptional initiation and elongation by RNA polymerase (Pol) II is associated with phosphorylation of its C-terminal tail (CTD). Depletion of Kin28, the TFIIH subunit that phosphorylates the CTD, does not affect elongation but causes Pol II occupancy profiles to shift upstream in a FACT-independent manner indicative of a defect in promoter escape. Stronger defects in promoter escape are linked to stronger effects on preinitiation complex formation and transcription, suggesting that impairment in promoter escape results in premature dissociation of general factors and Pol II near the promoter. Kin28 has a stronger effect on genes whose transcription is dependent on SAGA as opposed to TFIID. Strikingly, Kin28 depletion causes a dramatic increase in Mediator at the core promoter. These observations suggest that TFIIH phosphorylation of the CTD causes Mediator dissociation, thereby permitting rapid promoter escape of Pol II from the preinitiation complex. Display omitted •Kin28, the TFIIH kinase subunit, is important for promoter escape not elongation•Kin28 depletion dramatically increases Mediator occupancy at the core promoter•Stronger defects in promoter escape are linked to stronger transcriptional effects•TFIIH phosphorylation of the CTD causes Mediator dissociation and promoter escape Wong et al. show that depletion of Kin28, the TFIIH subunit that phosphorylates the RNA Pol II CTD, does not affect elongation but causes a defect in promoter escape and a dramatic increase in Mediator. Thus, TFIIH phosphorylation of the CTD causes Mediator dissociation, thereby permitting rapid promoter escape of Pol II from the preinitiation complex.