One of the outstanding problems of current observational cosmology is to understand the nature of sources that produced the bulk of the ionizing radiation after the Cosmic Dark Age. Direct detection of these reionization sources1 is practically infeasible at high redshift (z) due to the steep decline of intergalactic medium transmission2,3. However, a number of low-z analogues emitting Lyman continuum at 900 Å restframe are now detected at z < 0.4 (refs. 4–8) and there are also detections in the range 2.5 < z < 3.5 (refs. 9–14). Here we report the detection of Lyman continuum emission with a high escape fraction (>20%) from a low-mass clumpy galaxy at z = 1.42, in the middle of the redshift range where no detection has been made before and near the peak of the cosmic star-formation history15. The observation was made in the Hubble Extreme Deep Field16 by the wide-field Ultraviolet Imaging Telescope17 onboard AstroSat18. This detection of extreme ultraviolet radiation from a distant galaxy at a restframe wavelength of 600 Å opens up a new window to constrain the shape of the ionization spectrum. Further observations with AstroSat should substantially increase the sample of Lyman-continuum-leaking galaxies at cosmic noon.