Updated project metadata. Tumour DNA contains thousands of somatic single nucleotide variants (SNVs) in non-protein-coding elements, yet their functional significance remains poorly understood. Amongst the most highly mutated elements are long noncoding RNAs (lncRNAs), functional transcripts with known roles in carcinogenesis. To search for driver mutations in lncRNAs, we apply an integrative driver discovery algorithm to SNVs from 2583 primary tumours and 3527 metastases to reveal 54 potential “driver lncRNAs”. Our algorithm confirms a particularly high mutation rate in the iconic cancer lncRNA, NEAT1, which has been ascribed by recent studies to passenger effects. We directly test the functionality of NEAT1 SNVs using in cellulo mutagenesis, identifying discrete regions where mutations reproducibly increase cell proliferation in diverse cell backgrounds, both cancerous and normal. In particular, mutations in the 5’ region alter ribonucleoprotein assembly and boost the population of subnuclear paraspeckles, thus mechanistically linking mutations to cellular proliferation. We then used RNA-pull down followed by mass spectrometry to identify the protein interactor changing between the wild type and mutant form of NEAT1.