Urban land expansion can be driven by and drive population growth, but determining cause-effect relationships of urban land expansion and population growth is challenging due to the temporal resolution of decadal censuses for most countries. The relative timing of urban land expansion and population change was explored based on a case study for north Taiwan from 1990 to 2015. Data on urban land expansion were derived from a dense time series of Landsat satellite imagery, and population change from annual population registers at the district level. A novel algorithm for identifying the starting time of urban expansion was developed and tested based on dense time series of Vegetation-Impervious-Soil (V-I-S) proportion maps derived from Landsat surface reflectance imagery. The time of urbanization estimated by logistic regression was determined from the Impervious cover time series. The identified location and estimated time for newly urbanized lands were generally accurate, with 80% of urban expansion estimated within ± 2.4 years.