Kolonnawa Marsh (KM) is one of the important urban wetland ecosystems in Colombo, Sri Lanka, and plays an important role by providing invaluable ecosystem services. Owing to the continuous exploitation, and reclamation of home states and industries in recent decades, this urban wetland has experienced significant changes. To assess the spatiotemporal pattern of KM, land cover change (LC) from 2000 to 2022 and land use in 2022 were investigated. Land-use maps from the Survey Department of Sri Lanka in 2000, 2016 and Google Earth satellite images from 2000 to 2022 were developed to identify land cover; whereas land-use patterns in the KM were detected by drone data and analyzed with the Spatial Analyst extension of ArcGIS version 10.7. On the KM, marshy area, water-covered area, and area covered with vegetation were the dominant land cover features, and in the year 2000 marshy area covering by 65.81%, 55.69% in 2016, and 78.71% in 2022 of the total area respectively. However, from 2000 to 2016, the water-covered area increased from 34.19–44.31% (5.93 ha net gain), and 21.29% in 2016 to 022(16.2 ha net loss). In proportion to the total extent of KM, an area covered with vegetation increased by 24.25% in 2000, 29.98% in 2016 (3.43 ha net gain), and 44.99% (0.16 ha net gain) in 2022. Results of a drone survey done in the year 2022 has been revealed 9 land use features identified and accordingly, mixed woodland 37.66%, herb-dominated high vegetation 25.15%, highland vegetation associated with wetland 6.95%, herb dominated low vegetation 5.19%, stream reservation 0.78%. Thus, within a 300 m buffer zone from the margin of KM, 8.52% of the construction area, 40.39% of the built-up area, and 59.61% of home gardens respectively. Therefore, it can be concluded that the extent of the KM is decreasing over the study period while transforming into a terrestrial ecosystem. Hence, immediate interventions are needed to preserve this valuable freshwater ecosystem.