Herein, we determined the radioactivity concentration of isotopes in a ϕ1.5′′×1.5′′ LaBr3(Ce) crystal through a series of measurement experiments involving independent analytic approaches based on a single experimental setup. Experimental data was collected under self-triggering and double-coincidence modes operations inside a shielded chamber. During the self-triggering mode operation, the radioactivity concentration of 227Ac was estimated to be 0.00694 ± 0.00003 Bq/cm3 from the measurement of alpha particles using a pulse-shape discrimination method. During the double-coincidence mode operation, the radioactivity concentration of 138La was calculated from the double-coincidence measurement of electron capture and β-decay processes. The radioactivity concentration of 138La in the LaBr3(Ce) crystal was estimated to be 1.394 ± 0.008 Bq/cm3.