Weak lensing of type Ia supernovae (SNe Ia) is a systematic uncertainty in the use of SNe Ia as standard candles, as well as an independent cosmological probe, if the corresponding magnification distribution can be extracted from data. We study the peak brightness distribution of SNe Ia in the Pantheon sample and find that the high-z subsample shows distinct weak lensing signatures compared to the low-z subsample: a long tail at the bright end due to high magnifications and a shift of the peak brightness toward the faint end, consistent with findings from earlier work. We develop a technique to reconstruct the weak lensing magnification distribution of SNe Ia, p (μ ), from the measured SN Ia flux distribution and apply it to the Pantheon sample. We find that p (μ ) can be reconstructed at a significance better than 2 σ for the subsample of SNe Ia at z >0.7 (124 SNe Ia), and at a lower significance for the SNe Ia at z >0.9 (49 SNe Ia), due to the small number of SNe Ia at high redshifts. The large number of z >1 SNe Ia from future surveys will enable the use of p (μ ) reconstructed from SNe Ia as an independent cosmological probe.