The cathedral is dedicated to the martyred Saint Sebastian who is venerated by the Roman Catholic and Orthodox churches. St Sebastian is often portrayed in art and literature tied to a post or tree with arrows shot into his body. This is based on the legend about his lifetime in the former Western European region of Gaul at a time when Christians were persecuted by the Roman Emperor Diocletian.
While formerly considered the patron saint of the plague, St Sebastian is to date the patron saint of soldiers and athletes. St Sebastian's feast day is celebrated on 20 January, following the hoisting of the flag and vesper services on the preceding days.
St Sebastian's Cathedral is the seat of the Diocese of Mannar, installed in 1981.