The fair place to meet is Cleveland, OH — the city closest to the midpoint of Milwaukee and Washington. From the farther side that’s about 7 hr 1 min of driving.
Recommended midpoint
Cleveland, OH
From Milwaukee
7 hr 1 min
335 mi to Cleveland
From Washington
6 hr 22 min
304 mi to Cleveland
Washington has the shorter trip; the split is off by about 39 min. The alternatives below can even it out. Milwaukee and Washington are about 634 miles apart.
Milwaukee and Washington are about 634 miles apart by road. Split the difference and you arrive near Cleveland, the city closest to the halfway point between them. That puts roughly 7 hr 1 min of driving on the Milwaukee side and 6 hr 22 min on the Washington side — the fairest single meeting point among the cities near the middle.
That's a half-day drive from each side, so Cleveland suits an overnight or a weekend rather than a quick coffee — long enough to want a reason to stay, short enough to drive.
If Cleveland doesn't have what you're after, Columbus and Detroit are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between Milwaukee and Washington.
Cleveland is a city in the U.S. state of Ohio and the county seat of Cuyahoga County. Located along the southern shore of Lake Erie, it is situated across the lake from Ontario, Canada, and is approximately 60 miles west of the Ohio–Pennsylvania state line. Read more →
Milwaukee is the most populous city in the U.S. state of Wisconsin. It is located on the western shore of Lake Michigan at the confluence of the Milwaukee, Menomonee, and Kinnickinnic Rivers. Read more →
Washington, D.C., officially the District of Columbia and commonly known as simply Washington or D.C., is the capital city and federal district of the United States. Read more →
City descriptions adapted from Wikipedia (CC BY-SA); photos via Wikimedia Commons, credited above.
Estimates use straight-line distance and typical road speeds; real drive times vary with route and traffic.