In 1786, Francis Light of the British East India Company had landed on Penang Island, which at the time was also part of Kedah.
After brokering an agreement with the then Sultan of Kedah, Sultan Abdullah Mukarram Shah, regarding the cession of Penang Island to the British East India Company, Light returned to the island on 11 August that year and claimed it "in the name of His Britannic Majesty, King George III".

However, the British East India Company was slow to realise the importance of the hinterland across the Penang Strait from Penang Island (then named the Prince of Wales Island).
Merchant vessels traversing the Penang Strait were often threatened by pirates originating from the Perai River.
In addition, Kedah was being invaded by Siam and Sultan Abdullah, disgruntled by the lack of military aid as promised by Light, assembled his army in Seberang Perai in 1790 to retake the Prince of Wales Island.
The planned amphibious assault was thwarted by the British East India Company, which launched night attacks on Kedah's forts.

In 1800, the first Lieutenant-Governor of the Prince of Wales Island, Sir George Leith, gained a 189.3 km² (73.1 sq mi) strip of Seberang Perai from Sultan Abdullah and renamed it Province Wellesley.
The newly acquired land covered an area south of the Perai River.
Thus, at that point, the Perai River became the international border between the British-controlled Province Wellesley and Kedah.
At the time, Province Wellesley was heavily forested and sparsely populated, with a total population of only 5,000.

Province Wellesley was subsequently expanded thrice.
Following the Siamese conquest of Kedah in 1821, the British and the Siamese agreed in 1831 to demarcate the southernmost Siamese border with Province Wellesley along the Muda River; the British in effect enlarged Province Wellesley to include the area between the Muda River to the north and the Perai River to the south.
Province Wellesley's eastern boundaries with Siam was also delineated following another treaty in 1859.
The final change to Penang's boundaries was made in 1874 following the Pangkor Treaty between the British Empire and Perak, which included a clause on the demarcation of the Province Wellesley-Perak border south of the Kerian River.
Penang's borders have remained unchanged since then.
In 1786, Francis Light of the British East India Company had landed on Penang Island, which at the time was also part of Kedah. After brokering an agreement with the then Sultan of Kedah, Sultan Abdullah Mukarram Shah, regarding the cession of Penang Island to the British East India Company, Light returned to the island on 11 August that year and claimed it "in the name of His Britannic Majesty, King George III".
However, the British East India Company was slow to realise the importance of the hinterland across the Penang Strait from Penang Island (then named the Prince of Wales Island). Merchant vessels traversing the Penang Strait were often threatened by pirates originating from the Perai River. In addition, Kedah was being invaded by Siam and Sultan Abdullah, disgruntled by the lack of military aid as promised by Light, assembled his army in Seberang Perai in 1790 to retake the Prince of Wales Island. The planned amphibious assault was thwarted by the British East India Company, which launched night attacks on Kedah's forts.
In 1800, the first Lieutenant-Governor of the Prince of Wales Island, Sir George Leith, gained a 189.3 km² (73.1 sq mi) strip of Seberang Perai from Sultan Abdullah and renamed it Province Wellesley. The newly acquired land covered an area south of the Perai River. Thus, at that point, the Perai River became the international border between the British-controlled Province Wellesley and Kedah. At the time, Province Wellesley was heavily forested and sparsely populated, with a total population of only 5,000.
Province Wellesley was subsequently expanded thrice. Following the Siamese conquest of Kedah in 1821, the British and the Siamese agreed in 1831 to demarcate the southernmost Siamese border with Province Wellesley along the Muda River; the British in effect enlarged Province Wellesley to include the area between the Muda River to the north and the Perai River to the south. Province Wellesley's eastern boundaries with Siam was also delineated following another treaty in 1859. The final change to Penang's boundaries was made in 1874 following the Pangkor Treaty between the British Empire and Perak, which included a clause on the demarcation of the Province Wellesley-Perak border south of the Kerian River. Penang's borders have remained unchanged since then.
Seberang Perai