Seasonal abundance, daily activity, and behavior of adults of the robber fly Lasiopogon slossonae Cole and Wilcox were assessed with a capture-mark-recapture procedure for a population along 1 km of riverside in the Adirondack Mountains near Lake Placid, New York. Over the 2015 season, a total of 240 individuals were uniquely marked at this site; 73 were re-sighted at least one more time. The overall emergence window for this population in 2015 spanned 47 days with a maximum individual lifespan observed of 23 days; adult seasonal phenology periods for three other years are also summarized from presence/absence observations. In 2015, the adult sex ratio was slightly male-skewed (58%), but there was no significant difference between the sexes in home range size or perch distance from water. Flies packed more tightly into microhabitat patches as the overall abundance increased, and individuals showed some fidelity to relatively small home ranges (the median area of activity for marked individuals was 20 m²). However, longer dispersal also occurred rarely—the farthest movement we observed was 726 m over nine days. Individuals were active from morning to evening (usually 10:00 to 18:00) with a daily activity period that appeared to depend on both light intensity and temperature. Prey consisted mostly of Empididae, but also included Simuliidae, Anthomyiidae, and Cicadellidae. Both males and females were observed mating with multiple partners on different days.