Maryland Route 186 begins at Western Avenue on the District of Columbia boundary. The skewed intersection is about 500 ft (150 m) northeast of Western Avenue's intersection with Maryland Route 185 (Connecticut Avenue). The Washington, D.C. neighborhood of Chevy Chase lies to the south of the border while MD 186 starts north through Chevy Chase Village on the north side of the border. The road starts as a two-lane road with a speed limit of 25 mph (40 km/h), a status it maintains for its entire length.
MD 186 encounters its first four-way stop at Melrose Street. another stop sign at Oxford Street. another stop sign at Primrose Street. The road then begins to run along the border of the village of Martin's Additions at Quincy Street, while the road leaves Chevy Chase Village and enters Chevy Chase Section Three, which lies on the west side of the road, at Bradley Lane. Bradley Lane headed west leads to Maryland Route 191. MD 186 encounters its fourth stop sign at the second of two intersections with Raymond Street.
After the intersection with Turner Lane, MD 186 features its only non-residential properties on the east side of the road. These include the Martin's Additions Town Hall, a barber shop, a pharmacy, a small grocery store, a restaurant, and a few offices. The road encounters its fifth stop sign at Taylor Street. Shortly after Taylor Street, MD 186 leaves Martin's Additions and Chevy Chase Section Three and enters the village of Chevy Chase Section Five. MD 186 meets its sixth and final stop sign at Woodbine Street, then leaves Chevy Chase Section Five and enters the unincorporated Chevy Chasecensus-designated place. Soon afterwards, the road ends at Maryland Route 410.
Brookville Road predates the state road system as a link between Chevy Chase and the 7th Street Pike (now Georgia Avenue) near what is now Interchange 31 of the Capital Beltway, which led north to Brookeville. The other remaining sections of Brookville Road include the section of Maryland Route 410 between MD 186 and Jones Mill Road and an unnumbered section of Brookville Road in Silver Spring near the former National Park Seminary.
This particular part of Brookville Road was partially paved in 1910, while the road from its southern terminus to Thornapple Street was paved in 1927. The remainder of the road north to MD 410 was paved by 1930, when the adjacent section of the new East-West Highway was completed. The 1939 Maryland state highway map shows MD 186 assigned to its present course.