This study investigates the impact of varied daylight illuminance levels on user preferences and cognitive performance in offices, employing a virtual reality platform and HDRI 360-degree panorama images, whose illuminance level was validated using simulation. With 46 participants, a cognitive task known as the Stroop-test was conducted under nine illuminance levels, ranging up to 1500 lux. Additionally, participants were surveyed to determine their preferred horizontal illuminance level at desk height. The results uncovered distinct user preferences, with the majority of participants favoring illuminance levels above 700 lux, specifically 1100 and 790 lux, for reading and work-related tasks. Notably, none of the participants opted for illuminance levels below 300 lux, indicating these levels were deemed insufficient for their tasks. An analysis of cognitive task performance revealed significant differences between various illuminance levels. Generally, increasing the illuminance level in an office building will increase the office workers’ task performance. As illuminance levels exceeded 300 lux, participants exhibited enhanced performance in tasks such as reading words (RW), naming colors (NC), and total Stroop-test performance (TT). The 700–1500 lux can be considered suitable illuminance level for high-precision tasks, and 300–700 lux can be considered the medium illuminance in office environments. Based on these findings, an optimal illuminance range of 900–1100 lux is recommended for office environments, aligning with both user preferences and performance. This study offers valuable insights for architects and researchers in the development of daylighting design guidelines aimed at enhancing employees' cognitive capabilities and satisfaction.