Wildfires are an important disturbance in many biomes, influencing biodiversity, ecosystem function, and air quality. In areas where fires are rapidly changing, the largest impacts on ecosystems and humans are often caused by exceptionally large fires that spread rapidly and are often synchronized with nearby fires. While there is ample evidence that climate change is increasing burned areas in many regions of the world, we still lack understanding about the specific conditions that lead to extremely large fires. While most fires originate from a single ignition source and origin, multi-ignition fires, or fire complexes, arise from multiple distinct ignition points that grow together. Here, we investigate the genesis and impacts of multi-ignition fires across temperate and boreal biomes using satellite-derived fire event tracking datasets for California and the Arctic-boreal domain from 2012-2024. These datasets allowed us to track the growth of fire perimeters on 12-hourly time steps and to identify when and where fire events merge. Analysis of these data reveals that the largest fire in California (the August Complex Fire) originated from 13 separate ignition points and burned 4,489 km². Similarly, in Siberia, a wildfire in 2021 originated from 26 individual ignition points and burned 15,759 km². Over the 12-year study period, we found that fire complexes contributed disproportionally to regional burned areas, contributing to 50% of the total burned area in California and 59% in Arctic-boreal regions. We also find that multi-ignition fires largely drive the interannual variability in temperate and boreal regions and contribute significantly to large fire seasons. For example, multi-ignition fires contributed 70% of the burned area in California’s largest fire years (2020 and 2021), compared to only 22% in all other, smaller fire years since 2012. A combination of physical and management feedbacks may facilitate extreme fire growth and merging when multiple fires are ignited in close vicinity. On the physical side, we hypothesize that updrafts associated with intense fires may entrain surface winds that attract smaller fires and increase their rate of spread. Simultaneously, fires ignited from multiple locations can lead to especially dangerous situations for firefighting efforts, compelling incident commanders to prevent civilians and firefighters from being caught between several advancing fire lines, while also necessitating the triage of firefighting resources. Our findings aim to improve understanding of the influence of large-scale lightning storms in creating extremely large and destructive fire events.