Britain's Howell fired a final-round 5-under 67 on the Old Course at St. Andrews in Scotland, finishing 72 holes at 23-under 265.
He was waiting in the clubhouse as overnight leader Uihlein (69) narrowly missed a long birdie putt try on the 18th -- he also ended at 265 to force a playoff.
After both golfers parred the first playoff on 1st hole, play switched back to the 18th, where the American came up a bit short on a birdie try but Howell sank his attempt from 8 feet to win his first European Tour event in seven years.
Tom Lewis fired an 8-under 64 Sunday to grab a share third of place Shane Lowry (68) at 22-under 266.
Tommy Fleetwood (67) claimed sole possession fifth at minus-21 while Garth Mulroy (68) ended in sixth one shot further back.