Transcriptional reactivation of TERT, the catalytic subunit of telomerase, is necessary for cancer progression in about 90% of human cancers. The recent discovery of two prevalent somatic mutations—C250T and C228T—in the TERT promoter in various cancers has provided insight into a plausible mechanism of TERT reactivation. Although the two hotspot mutations create a similar binding motif for E-twenty-six (ETS) transcription factors, we show that they are functionally distinct, in that the C250T unlike the C228T TERT promoter is driven by non-canonical NF-κB signalling. We demonstrate that binding of ETS to the mutant TERT promoter is insufficient in driving its transcription but this process requires non-canonical NF-κB signalling for stimulus responsiveness, sustained telomerase activity and hence cancer progression. Our findings highlight a previously unrecognized role of non-canonical NF-κB signalling in tumorigenesis and elucidate a fundamental mechanism for TERT reactivation in cancers, which if targeted could have immense therapeutic implications.