Mackay is a town in Custer County, Idaho, United States. The population was 439 at the 2020 census. The town is pronounced "Mackie" with t…Mackay is a town in Custer County, Idaho, United States. The population was 439 at the 2020 census. The town is pronounced "Mackie" with the accent on the first syllable. The town was named in honor of John William Mackay who owned the local copper mines. The nearby Mackay Peak in the White Knob Mountains is also named after him.