Brief History of Districts in Sri Lanka.

In 1833 a legislative council was created, making the island a politically and administratively single unit. Five provinces were created, later expanded into nine, and these were subdivided into twenty-one districts. These districts were administered by officials known as Government Agents or Assistant Government Agents.

In 1955, the district replaced the province as the country's main administrative unit. Ampara District was created in April 1961, followed by the creation of Mullaitivu and Gampaha districts in September 1978 through a new constitution, which also reintroduced the province as the main administrative unit. The last district to be created was Kilinochchi in February 1984, and the current constitution (that of 1978) states that the territory of Sri Lanka consists of 25 administrative districts. These districts may be subdivided or amalgamated by a resolution of the Parliament of Sri Lanka.

Since 1987 the 25 districts are purely administrative under the 9 Provincial Councils created in by the 13th Amendment to the Sri Lanka constitution.

Note that although the Northern Province has Five Administrative Districts, it has only two Electorial Districts. Therefore there are only 22 Electorial Districts in Sri Lanka.
Jaffna and Kilinochchi form the Jaffna Electorial District.
Mannar, Vavuniya, and Mullaitivu form the Vanni Electorial District.
The Ampara District is known as the Digamadulla Electorial District.

Current Statistics of 25 Districts in 9 Provinces of Sri Lanka.

District
Capitol		Province Area
sq.km		Popula.
density
Jaffna Northern 1,025 628
Kilinochchi Northern 1,279 94
Mannar Northern 1,996 53
Mullaitivu Northern 2,617 38
Vavuniya Northern 1,967 92
 
Puttalam NorthWestern 3,072 264
Kurunegala NorthWestern 4,816 348
 
Gampaha Western 1,387 1,711
Colombo Western 699 3,417
Kalutara Western 1,598 772
District
Capitol		Province Area
sq.km		Popula.
density
Anuradhapura NorthCentral 7,179 128
Polonnaruwa NorthCentral 3,293 131
 
Matale Central 1,993 247
Kandy Central 1,940 715
Nuwara Eliya Central 1,741 414
 
Kegalle Sabaragamuwa 1,693 497
Ratnapura Sabaragamuwa 3,275 334
District
Capitol		Province Area
sq.km		Popula.
density
Trincomalee Eastern 2,727 150
Batticaloa Eastern 2,854 201
Ampara Eastern 4,415 153
 
Badulla Uva 2,861 287
Monaragala Uva 5,639 81
 
Hambantota Southern 2,609 239
Matara Southern 1,283 637
Galle Southern 1,652 655
 
Total 65,610 323

Province Capitol CreatedArea
km2		Population Districts
1955 1985		Popula.
Density
Northern Jaffna 1833 8,290 1,058,762 3 5 128
North WesternKurunegala 1845 7,506 2,370,075 2 2 316
Western Colombo 1833 3,593 5,821,710 2 31,620
North CentralAnuradhapura1873 9,741 1,259,567 2 2 129
Central Kandy 1833 5,575 2,558,716 3 3 459
Sabaragamuwa Ratnapura 1889 4,921 1,918,880 2 2 390
Eastern Trincomalee 1833 9,361 1,551,381 2 3 166
Uva Badulla 1886 8,335 1,259,900 2 2 151
Southern Galle 1833 5,383 2,464,732 3 3 458
Total 62,70520,263,7232125 323