It affected the Tahoe and Eldorado National Forests and destroyed 78 structures in the rural communities of Michigan Bluff, Foresthill, and Volcanoville.

The precise cause of the fire is not known, but the possible role of Pacific Gas & Electric Company (PG&E) equipment is the subject of multiple civil lawsuits and a Forest Service investigation.

[12] The fire continued to burn actively through the night, moving northwest up the side of the Middle Fork American River canyon.

[16] An elderly couple was trapped behind the fire line when their vehicle became disabled and a wild pig attacked their dogs, necessitating a sortie through the flames by an El Dorado County Sheriff's deputy.

[23][24] This cyclonic vortex within the rotating column of smoke was connected to the ground, had a circulation extending up to approximately 10,000 feet (3,000 m), and generated winds equivalent in strength to an EF-1 tornado.

[26][27] On September 9, firefighting conditions improved with slightly cooler temperatures and low winds, causing the fire to remain under an inversion of its own smoke.

[31] On September 13, remaining tropical moisture from the remnants of Hurricane Kay left the area and was replaced by persistent southwest winds.

Though firefighters achieved 18% containment by the morning, the winds allowed smoke to clear and the inversion to break, leading to an uptick in fire activity.

The spot fire grew aggressively and became established, creating a large plume as it burned up towards Foresthill, which had been under a mandatory evacuation for several days by that point.

[33] A massive airshow attempted to halt the fire, as 16 different aircraft, including all four of the Very Large Air Tankers operated in the United States, aided ground crews.

[39] On September 19, for the first time since igniting, the Mosquito Fire did not increase in acreage, though smoking hot spots were still visible during gaps in the rain.

At least two firefighters were injured; one hurt their wrist in a fall and another stepped into a still-burning stump hole, sustaining second-degree burns on their leg.

[58] Smoke from the Mosquito Fire routinely led to hazardous air qualities in many nearby regions in Northern California and Nevada, including the Lake Tahoe Basin and the cities of Folsom and Auburn.

[59][13][60] On at least one day, schools throughout Washoe County were forced to close as local health officials declared an air quality emergency.

[65][66] The Mosquito Fire also burned large parts of multiple watersheds that empty into Oxbow Reservoir and the Middle Fork American River, providing drinking water to communities in the Central Valley and Sierra foothills.

High soil burn severity areas are more prone to increased runoff rates and erosion, creating higher probabilities for downstream flooding and debris flows.

[74][75] On November 19, Governor Newsom signed an executive order allowing some environmental regulations to be suspended to expedite the removal of hazardous debris and speed other fire recovery actions.

[77][78] As of 2023, the company is cooperating with an investigation by the Forest Service and the Department of Justice into the cause of the blaze, and has been asked to produce documents and information for the probe.

[80] On September 8, PG&E submitted an incident report to the California Public Utilities Commission (CPUC), indicating that "electrical activity occurred close in time to the report time of the fire", and that the U.S. Forest Service had placed caution tape around one of their 60-kilovolt overhead power transmission poles, on the Oxbow Tap/Middle Fork #1 line.

[82][83] In a September 8 interview, PG&E executive vice president Sumeet Singh said that the power line on the transmission pole tripped offline in what was described as a fault, close to the fire's reported start time.

A preliminary inspection conducted after the fire started showed that the power line remained hung on the steel transmission pole, with no signs of contact with a tree.

The 2021 Dixie Fire, California's single largest wildfire, began in Butte County when a tree fell on a PG&E power line in the Feather River Canyon.

The 2018 Camp Fire, California's deadliest wildfire, also began in the Feather River Canyon when hardware on a PG&E power line failed.

[88] In late September 2022, two civil lawsuits were filed against PG&E in the San Francisco Superior Court, both alleging that the fire was ignited by the company's utility infrastructure and a failure to safely operate/maintain it.

[89][90][91] In December 2022, the Placer County Water Agency filed another lawsuit, seeking damages from PG&E related to the Mosquito Fire.

[48] In January 2023, El Dorado and Placer counties (with the El Dorado Water Agency, Georgetown Divide Public Utilities District and Georgetown Divide Fire Protection District as co-plaintiffs) filed a third suit against PG&E, again accusing the company of negligence and seeking to recoup costs from "fire suppression; law enforcement costs and overtime; administration, funding and operation of emergency operations and evacuation shelters; and lost tax revenue.