Lake Mason National Wildlife Refuge consists of three separate tracts of land: Lake Mason Unit, Willow Creek Unit, and North Unit. 

To access the 13,589-acre Lake Mason Unit, turn south from Highway 87 on 4th Street West in Roundup. Proceed 1 block and turn west on the Golf Course Road. Continue for 6.5 miles, then turn north and proceed 2 miles to the refuge boundary.

The 2,240-acre Willow Creek Unit is 22 miles northwest of Roundup, Montana. To access the Willow Creek Unit, proceed 11 miles north of Roundup on Highway 87. Turn west on Snowy Mountain Road (also Forest Service access to the Little Snowy Mountains), and drive 13.8 miles to the unit.

The 5,323-acre North Unit is located 24-miles northwest of Roundup, Montana. For access to the North Unit, drive north of Roundup 11 miles on Highway 87, and turn west on the Snowy Mountain Road (also Forest Service access to the Little Snowy Mountains). After 7 miles, turn north on the Graves Road and continue 7.3 miles, then turn west again on a small, two-track trail. The refuge lies 2.2 miles west of the Graves Road.