The Mu2e experiment at Fermilab will search for charged-lepton flavour violating neutrino-less conversion of negative muons into electrons in the coulomb field of Al nuclei. The conversion electron has a monoenergetic 104.967 MeV signature slightly below the muon mass and will be identified by a high-resolution tracker and an electromagnetic calorimeter (EMC), reaching a single event sensitivity of about 3 × 10−17, four orders of magnitude beyond the current best limit. The calorimeter is composed of 1348 pure CsI crystals, each read by two custom UV-extended SiPMs, arranged in two annular disks. The EMC has high granularity, 10 % energy resolution and 500 ps timing resolution for 100 MeV electrons and will need to maintain extremely high levels of reliability and stability in a harsh operating environment with high vacuum, 1 T B-field and radiation exposures up to 100 krad and 1012 n1MeV/cm2. The calorimeter design, along with the custom front-end electronics, cooling and mechanical systems were validated through an electron beam test on a large-scale 51-crystals prototype (Module-0). Extensive test campaigns were carried out to characterise and verify the performance of crystals, photo-detectors, analogue and digital electronics, including hardware stress tests and irradiation campaigns with neutrons, protons, and photons. The production and quality control phases of all calorimeter components is about to be completed and the assembly of the downstream calorimeter disk is at an advanced state. A full vertical slice test with the final electronics was carried out on the Module-0, along with the implementation and validation of the calibration procedures.