The original status of Phra Nakhorn Si Ayutthaya City Municipality was an ancient city sanitation that was established according to the Royal command promulgated in the Act of legislation regarding the sanitation management of a provincial town Bangkok Era 127. On 25 May 2459 (1916) ,the ancient city sanitation was located in the area of a district of the ancient city that is presently known in the name of Phra Nakhorn Si Ayutthaya District, together with five sub-districts in its supervision comprising Hua Ror, Rattanachai, Tha wasukri, Klong Suanplu and Kamueng Sub-districts.
Later,in B.E. 2478 (1935) , there was a Royal decree to enhance the status of the ancient city sanitation to be Muang Nakhorn Si Ayutthaya Municipality as well as to expand its precinct according to the Royal decree for setting up Nakhorn Si Ayutthaya Muang Municipality B.E. 2498-2499 and 2505 respectively. In B.E. 2542 , there was a Royal decree to lift up the status of Nakhorn Si Ayutthaya Muang Municipality to be Nakhorn Si Ayutthaya Nakhorn Municipality, and later in B.E. 2548 the Royal decree for changing the name from Nakhorn Si Ayutthaya Nakhorn Municipality to Phra Nakhorn Si Ayutthaya Nakhorn Municipality has been enacted so far.
The official emblem of Phra Nakhorn Si Ayutthaya Nakhorn Municipality is a castle , and inside the castle there is a conch-shell laid on Waen Fa tray. Also , the castle is under a tree called Mun in Thai . The exact meaning of the emblem of the municipality comes from the chronicle of building Nakhorn Si Ayutthaya town. Legend had it that in B.E. 1890 there happened an outbreak of contagious epidemics or plague, and a large number of the people died. Therefore , King U-Thong evacuated from the old town to settle down in a new one at Nong Sanho sub-district , surrounded by the rivers. Here , he ordered his entourage to set up temporary shrine for propitiating ceremony. While a burning Brahminic ceremony to level ground for the town , what happened was that the entourage , after digging under the Mun tree , saw a pure circumambulatory conch-shell. King U-Thong was very delighted with that auspicious omen , so he let his accompanying persons build up a small castle for housing the aforesaid conch-shell.