Garrett County was the last county created in Maryland, carved out of Allegany County in 1872 and named for John Work Garrett (1820-1884), the president of the Baltimore and Ohio Railroad. The B&O had pushed track through these mountains a generation earlier, and Garrett used his political connections to ensure the new county was named after the man who had made it economically viable. The county is the westernmost in Maryland, entirely within the Appalachian Mountains, sharing borders with four West Virginia counties to the south and west and with Pennsylvania along the Mason-Dixon line to the north. The eastern boundary with Allegany was defined by the Bauer Report, submitted to Governor Lloyd Lowndes Jr. on November 9, 1898 - a reminder that even on the map this corner of Maryland was settled later than the rest.

The county sits entirely within the Allegheny Plateau, the highland zone where the Appalachian Plateau meets the Eastern Continental Divide. Four flat-topped ridges define the high country, reaching their maximum at Hoye-Crest. River valleys cut narrow and deep, with ravines 1,000 to 1,800 feet below the surrounding peaks. The Savage River, a Potomac tributary, drains roughly a third of the county. The Casselman and Youghiogheny rivers drain the rest, flowing north into Pennsylvania. The Glades - a 601-acre peatland with bog layers up to 9 feet thick - is fed entirely by rainwater and ranks as one of the oldest mountain peatlands in the Appalachians. Just north of Bittinger, along Maryland Route 495 near the Casselman River, low mounds of wind-blown silt remain from a glacial lake that filled the valley during the Pleistocene - loess dunes, rare in this part of North America.

Since Garrett County was created in 1872, no Democrat has ever carried it in a presidential election. It is one of just 15 counties in the United States with that record, most of them old Unionist strongholds in formerly slave states. The only exception came in 1912, when the Republican split between William Howard Taft and Theodore Roosevelt's Bull Moose Party let Roosevelt take the county. In modern Maryland, where statewide politics has trended sharply Democratic, Garrett has become the lone holdout. In 2022 it was the only county in the state to vote against legalizing recreational cannabis. In 2024 it was the only county to vote against the abortion-rights amendment to the state constitution. The 2020 census counted 28,806 residents - the third-smallest population of any Maryland county - and 84.2 percent of them live in rural areas.

Garrett County's modern economy runs on tourism. Deep Creek Lake, a 3,900-acre reservoir created in 1925 for a hydroelectric project, draws summer visitors from Washington, Baltimore, and Pittsburgh. The Wisp ski resort in McHenry is Maryland's only commercial alpine ski area, climbing the western face of Marsh Mountain. Over 76,000 acres of state parks and public forest land - Big Run, Casselman River Bridge, Deep Creek Lake, Herrington Manor, New Germany, Sang Run, Swallow Falls - sprawl across the county. Garrett County is the only county in Maryland that produces natural gas. It also hosts the only Amish community in the county, a New Order district of about 70 homes near Oakland that traces its roots to 1850 and permits electricity inside its houses. The southernmost edge of the county falls within the United States National Radio Quiet Zone, a 1958 federal designation that limits radio emissions to protect the Green Bank Telescope just across the West Virginia line.