The Marquardt least-squares method is applied in this paper for estimation of the Pulse function’s non-linear parameters in order to approximate measured lightning currents. Such procedure is generalized so it could be used for other pulse functions representing lightning current waveshapes. The obtained results show that this method provides good results for the IEC 62305-1 standard lightning current waveshapes of the first and subsequent return stroke currents, and also for some measured fast- and slow-decaying current waveshapes.