MD 198 begins at a four-way intersection between MD 650 New Hampshire Avenue and Norbeck Road (an at-grade extension of MD 28 from its eastern terminus at MD 182), several miles to the south of Ashton. Originally, Norbeck Road did not exist at this intersection, and MD 198 ended at a small three-way intersection with MD 650 several hundred yards to the north.
East of here, MD 198 is a busy 40 mph (64 km/h) suburban/rural arterial known as Spencerville Road, carrying traffic through central Montgomery County. Three miles to the east, in downtown Burtonsville, it encounters Old Columbia Pike, the original alignment of US 29, and also formerly signed as MD 196. At this intersection, MD 198 assumes the name of Old Columbia Pike. A short distance further east, the route encounters a second alignment of US 29, known as Columbia Pike, that was built in the late 1960s to carry the U.S. route around the Burtonsville Crossing Shopping Center. After the completion of roadworks on December 7, 2005, this alignment of US 29 is out of use as a through mainline route and now serves merely as a local arterial for the community; the segment north of MD 198 ends at Dustin Road, while the segment south was removed. Slightly further to the east, MD 198 encounters the current alignment of US 29 through Burtonsville. This alignment is much straighter than the previous alignment and links with MD 198 via a grade-separated 3/4 diamond interchange.
Beyond the route's interchange with US 29, MD 198 widens into a four-lane non-limited access arterial and carries moderate to heavy amounts of traffic on an east–west alignment. Here, it is known as Sandy Spring Road, and passes to the north of a major industrial complex and electrical substation before meeting I-95. The two routes interchange via a large, high-speed partially unrolled cloverleaf, with the connection between MD 198 west and I-95 south being served via a semidirectional ramp. In the northeast quadrant of the interchange, Old Sandy Spring Road (a former alignment of MD 198) begins at a dead end and parallels the current routing eastward.
Beyond I-95, MD 198 maintains its four-lane non-limited access cross-section as it passes a major branch office of Tower Federal Credit Union as it enters Laurel. At 9th St. in downtown Laurel, MD 198 splits into a one-way couplet; the eastbound component is known as Gorman Avenue, and the westbound component is known as Talbott Avenue. Heading east, the couplet crosses the southern terminus of MD 216 (7th St.) and US 1, itself a couplet.
At US 1, the couplet collapses into a normal four-lane non-limited access carriageway as it crosses the B&O Capital Subdivision (also known as the MARC Camden Line) and Lafayette Road. Beyond the crossing, MD 198 is known as Fort Meade Road, and meets the northern terminus of MD 197 several hundred yards east of the B&O. Past MD 197, MD 198 resumes its four-lane non-limited access cross section and again serves as a major arterial, carrying heavy suburban and commuter traffic between Laurel and points east. Near the route's crossing of the Patuxent River, the route connects with Laurel Race Track Road, which serves the Laurel Park Racecourse. Continuing east, the route crosses several suburban and rural roads before meeting the Baltimore-Washington Parkway at a modified cloverleaf interchange.
Beyond the B-W Parkway, MD 198 narrows to a fast two-lane highway known locally as Laurel-Fort Meade Road and continues due east, passing an old alignment of MD 216 and crossing the Little Patuxent River near a water treatment plant before veering away from its original alignment to run parallel to MD 32 for a short distance. Eventually, MD 198 reaches a dumbbell interchange with MD 32. Beyond this interchange, MD 198 becomes unsigned and winds its way to the entrance to Fort Meade, where it terminates; the roadway continues into the fort as Mapes Rd., eventually reaching MD 175.