This work presents two searches for a high-mass Higgs boson in the H-->WW-->lvqq decay channel using the ATLAS detector to analyze the high-energy proton-proton collisions provided by the Large Hadron Collider at two different center-of-mass energies, sqrt(s) = 8 TeV in 2012 and sqrt(s) = 13 TeV in 2015, corresponding to two independent datasets with sizes given by their integrated luminosities of 20.3 fb-1 and 3.2 fb-1, respectively. No significant excess of data above the expected background is observed in either analysis, so upper limits are set on the production cross-section times branching ratio, as a function of the hypothesized boson mass, for the various signal models tested. The derived limits substantially improve upon previous results.