Most of the burned area was public lands administered by the United States Forest Service, much of it in the Three Sisters Wilderness.

During the afternoon of August 11, 2017, a wildfire was reported in the Deschutes National Forest, 9 miles (14 km) west of Sisters.

The forest area on the eastern slopes of the Cascade Range, where the Milli Fire began, is dominated by ponderosa pine.

Initially, it was allowed to burn because it was in a remote part of the wilderness and firefighting crews were already deployed on other higher priority fires.

As a result, fire officials announced that their highest priority was protecting homes and private property south and west of the community.

Nevertheless, firefighters made progress building fire lines with the help airtankers and helicopter water drops.

In addition, approximately 1,600 residents of the Sisters area were told to prepare to evacuate their homes in case the fire began to burn north toward the community.

That evening, Forest Service fire managers and local officials briefed members of the Sisters community at a town meeting.

[6][8][9][10] On August 18, the fire continued to burn actively through thick forest and brush covered areas with standing dead timber left over from two previous large wildfires.

During the day, firefighters continued to work fire lines on the north and east flanks of the blaze, trying to protect private property and the Whychus Creek watershed.

Crews cleared away ground fuels using heavy equipment and burn-out operations widened fire breaks.

In the late afternoon, crews prepared to do burnout work on the northwest side of the fire if nighttime weather condition were favorable.

[20] On August 25, morning burnout operations were delayed due to poor visibility caused by heavy smoke.

[21][22][23] Over the weekend of August 26–27, warm-dry weather encouraged the fire along the west side as it moved through the Three Sisters Wilderness toward Lava Camp Lake.

During this period, it began burning through some private timber land inside the fire line that had previously escaped the flames.

[28][29] When the new leadership team arrived on August 31, the Milli Fire covered 22,190 acres (89.8 km2) and was still only 44 percent contained with 451 firefighters on site.

Unburned forest within the fire's perimeter continued to burn including a large block of private timber land.

In most areas the fire crept slowly across the forest floor with only a few isolated pocket where larger tree were torched off.

[28][30] The fire continued to burn slowly down slope through thick timber in the Three Sisters Wilderness toward Route 242 on September 1–2.

Several pumper trucks along with ground crew worked around Lava Camp Lake to mop-up hot spots and protect the campground infrastructure.

The Forest service worked with the Oregon Department of Transportation to remove logs, rock, and other debris from the roadway.

Helicopters made water drops to help ground crew mop-up hot spots in the private forest land near the north edge of the fire and along Route 242.

On the southwest side, the fire kept on burning slowly down slope through the Three Sisters Wilderness Area, occasionally torching pockets of taller trees.

On that date, the Forest Service estimated the Milli Fire covered 24,025 acres (97.23 km2) with containment remaining at 60 percent.

Forest Service and Oregon Department of Transportation crews also removed hazardous trees from the Route 242 corridor and repaired culverts along the roadway.

This activity was expected to continue until a season ending rain or snow fall naturally extinguished the last pockets of fire.

However, Route 242 remained closed while Oregon Department of Transportation and Forest Service crews worked to remove hazardous roadside snags.

On steep slopes, some large trees were cut down and left lying across the hill side to help prevent soil erosion.

On slopes above roads, high stumps were left exposed to help prevent burned logs from rolling down onto roadways.

[49][50] As a follow-up to the initial post-fire assessment, the Forest Service began preparing a detailed resource rehabilitation and protection plan for the Milli Fire area.