La Grandière was promoted to Rear Admiral in December 1861, and in 1862 was Major General in Cherbourg and then in Brest.

Tự Đức, the Nguyễn emperor of Vietnam, ceded control of Saigon and the adjoining provinces of Biên Hòa, Gia Định and Định Tường to the French under the Treaty of Saigon of 5 June 1862, which they organized as the colony of Cochinchina.

He also gave the French control of his trade and foreign relations and accepted French advisers, in effect becoming a French puppet.

Admiral Louis Adolphe Bonard ensured that in the Treaty of Hué of 14 April 1863 Tự Đức confirmed the 1862 Treaty of Saigon under which Tonkin became a protectorate.

In May 1863 Prosper de Chasseloup-Laubat, Minister of Marine and Colonies, named La Grandière Governor General and commander of the Cochinchina naval station, with the Duperré as his flagship.

La Grandière proved to be a capable administrator and economist.

He developed the port of Saigon with a 72 metres (236 ft) refit basin, a 92 metres (302 ft) floating dock and various other facilities to make the port a major site for reception and repair of ships.

At first he prohibited the export of rice to ensure the supply of the local population and improve crops so they could again be exported.

He developed French and indigenous administrations, created more schools, recruited and trained Indochinese troops and abolished corporal punishment.

He supported Ernest Doudart de Lagrée and Francis Garnier in their Mekong expedition of 1866–1868.

In 1866 La Grandière initiated collection of artifacts from ancient Khmer and Chàm sites to be exhibited in Saigon, but they had to be stored until a suitable building could be made available.

La Grandière made Achille-Antoine Hermitte head of his architectural department, as recommended by admirals Pierre-Gustave Roze and Gustave Ohier.

His priority was to design a new Governor's Palace since the existing wooden building was in disrepair.

La Grandière laid the cornerstone for this building on 23 March 1868, a block of blue granite from Biên Hòa containing a lead coffer that in turn contained newly-minted gold, silver and copper coins of Napoleon III.

In 1868 La Grandière gave precise orders for collection and classification of dossiers, the origin of the French archives in Indochina.

He recruited M. A. Petiton, a mining engineer from La Grand-Combe, Gard, as Chief Engineer of the Cochinchina Mines Service, with the mandate of undertaking geological and mining studies throughout Indochina.

Petiton arrived in October 1868, after La Grandière had left Saigon.

On 11 August 1863 Admiral de La Grandière signed a Treaty of Friendship, Trade and French Protection with King Norodom of Cambodia.

La Grandière visited Siem Reap and the ruins of Angkor Wat, then in Siamese territory, but he treated it as if it were part of Cambodia.

La Grandière was promoted to Vice Admiral in September 1865.

He supervised the 1866 French campaign against Korea by Rear Admiral Roze following the massacre of several French Catholics by the regent of the Kingdom of Korea.

La Grandière was concerned that the Vietnamese provinces to the west of Cochinchina would be trouble spots, and secretly organized an expedition that occupied Vĩnh Long, Sa Đéc, Châu Đốc and Hà Tiên on 20–24 June 1867.

Phan Thanh Giản, former envoy of Tu Duc in Paris and now governor of these provinces, told the mandarins to avoid bloodshed and submit.

He then committed suicide.

The French now controlled the whole of southern Vietnam.

La Grandière wrote,
I am attached to Cochinchina as to a child of which I took care during an illness and which gave me much concern... Yes, we shall maintain this glory, despite of and against our detractors.
Yes, we will guide this colony, slowly perhaps, but with the assurance of its brilliant future.
And we shall make ourselves loved and respected by our neighbours, by our Annamite subjects, and what is the most difficult of all, even by our French merchants.

La Grandière returned to France in April 1868.

He was named Maritime Prefect of Toulon in 1870.

He retired to Finistèr
La Grandière was promoted to Rear Admiral in December 1861, and in 1862 was Major General in Cherbourg and then in Brest.
Tự Đức, the Nguyễn emperor of Vietnam, ceded control of Saigon and the adjoining provinces of Biên Hòa, Gia Định and Định Tường to the French under the Treaty of Saigon of 5 June 1862, which they organized as the colony of Cochinchina.
He also gave the French control of his trade and foreign relations and accepted French advisers, in effect becoming a French puppet.
Admiral Louis Adolphe Bonard ensured that in the Treaty of Hué of 14 April 1863 Tự Đức confirmed the 1862 Treaty of Saigon under which Tonkin became a protectorate.
In May 1863 Prosper de Chasseloup-Laubat, Minister of Marine and Colonies, named La Grandière Governor General and commander of the Cochinchina naval station, with the Duperré as his flagship.
La Grandière proved to be a capable administrator and economist.
He developed the port of Saigon with a 72 metres (236 ft) refit basin, a 92 metres (302 ft) floating dock and various other facilities to make the port a major site for reception and repair of ships.
At first he prohibited the export of rice to ensure the supply of the local population and improve crops so they could again be exported.
He developed French and indigenous administrations, created more schools, recruited and trained Indochinese troops and abolished corporal punishment.
He supported Ernest Doudart de Lagrée and Francis Garnier in their Mekong expedition of 1866–1868.
In 1866 La Grandière initiated collection of artifacts from ancient Khmer and Chàm sites to be exhibited in Saigon, but they had to be stored until a suitable building could be made available.
La Grandière made Achille-Antoine Hermitte head of his architectural department, as recommended by admirals Pierre-Gustave Roze and Gustave Ohier.
His priority was to design a new Governor's Palace since the existing wooden building was in disrepair.
La Grandière laid the cornerstone for this building on 23 March 1868, a block of blue granite from Biên Hòa containing a lead coffer that in turn contained newly-minted gold, silver and copper coins of Napoleon III.
In 1868 La Grandière gave precise orders for collection and classification of dossiers, the origin of the French archives in Indochina.
He recruited M. A. Petiton, a mining engineer from La Grand-Combe, Gard, as Chief Engineer of the Cochinchina Mines Service, with the mandate of undertaking geological and mining studies throughout Indochina.
Petiton arrived in October 1868, after La Grandière had left Saigon.
On 11 August 1863 Admiral de La Grandière signed a Treaty of Friendship, Trade and French Protection with King Norodom of Cambodia.
La Grandière visited Siem Reap and the ruins of Angkor Wat, then in Siamese territory, but he treated it as if it were part of Cambodia.
La Grandière was promoted to Vice Admiral in September 1865.
He supervised the 1866 French campaign against Korea by Rear Admiral Roze following the massacre of several French Catholics by the regent of the Kingdom of Korea.
La Grandière was concerned that the Vietnamese provinces to the west of Cochinchina would be trouble spots, and secretly organized an expedition that occupied Vĩnh Long, Sa Đéc, Châu Đốc and Hà Tiên on 20–24 June 1867.
Phan Thanh Giản, former envoy of Tu Duc in Paris and now governor of these provinces, told the mandarins to avoid bloodshed and submit.
He then committed suicide. 
The French now controlled the whole of southern Vietnam.
La Grandière wrote,
I am attached to Cochinchina as to a child of which I took care during an illness and which gave me much concern... Yes, we shall maintain this glory, despite of and against our detractors. Yes, we will guide this colony, slowly perhaps, but with the assurance of its brilliant future. And we shall make ourselves loved and respected by our neighbours, by our Annamite subjects, and what is the most difficult of all, even by our French merchants. 
La Grandière returned to France in April 1868.
He was named Maritime Prefect of Toulon in 1870.
He retired to Finistèr
Pierre-Paul de La Grandière