Driven by gravity, galaxies are expected to continuously grow through the merging of smaller systems. To derive their past merger history is challenging, as the accreted stars disperse quickly; yet, it is a needed step to test the theory of hierarchical evolution. The merger histories of the most massive Local Group spirals, the Milky Way and M31, have been re- cently uncovered by using the motion and chemistry of their individual stars. On the other hand, the details of the merger history of galaxies at larger distance have so far remained hidden. Here we report the discovery of an ancient, massive merger event in the lenticu- lar galaxy NGC 1380 in the Fornax cluster. By applying a recently developed population-orbital superposition model (Zhu at al 2020) to NGC 1380’s surface brightness as well as stellar kinematic, age, and metallicity maps from VLT/MUSE IFU data (Sarzi et al 2018), we obtain the stellar orbits, age and metallicity distributions of this galaxy. The highly radial orbits which make up an inner stellar halo are ∼ 13 Gyr old with metallicity Z/Z⊙ ∼ 1.2 and comprise a stellar mass of M∗,halo(r<2Re)∼3.4×10^10 M⊙. By comparing to analogues from the cosmological galaxy simulation TNG50 (Pillepich 2019), we find that the formation of the inner stellar halo of NGC 1380 requires a merger with a massive satellite galaxy with stellar mass of ∼ 3 × 10^10 M⊙ that occurred roughly ∼ 10 Gyr ago. Moreover, we infer the total accreted stellar mass of NGC 1380 to be ∼ 6 × 10^10 M⊙. The massive merger in NGC 1380 is the first major merger event found in a normal phase-mixed galaxy beyond the Local Volume, and it is the oldest and most massive one identified in nearby galaxies so far. Our chemo-dynamical method, when applied to extended deep IFU data and in combination with cosmological galaxy simulations, can quantitatively unravel the merger history of a large number of nearby galaxies.