The Black Cats fielded a side with several first-team regulars but fell behind in the eighth minute to the League One side when Howard finished from close range.
Sunderland were rather subdued in the first half although improved after the interval, with David Meyler shooting just wide from 25 yards.
Their best chance of a leveller came when Fraizer Campbell saw his stooping header superbly tipped over the bar by Scott Flinders as Sunderland crashed to their second pre-season defeat.
St Mirren suffered a 1-0 defeat against League One Carlisle United in their penultimate pre-season friendly on Friday night.
Lee Miller scored the only goal of the game midway through the second half.