The Irishman moved to Hull in January and was the scourge of his ex-employers at the KC Stadium, winning a contentious penalty before scoring his team's second as Steve Bruce's men eased their relegation worries with a much-needed three points against the struggling visitors.
Long seemed to search for contact with Craig Dawson in the 32nd minute, but referee Chris Foy deemed it a foul and pointed to the spot.
Ben Foster saved Nikica Jelavic's kick, only for Liam Rosenior to follow up and head in his first goal for the club.
The win was Hull's first at home in the league this year and it deepened West Brom's woes, leaving Pepe Mel's side looking nervously over their shoulders at the bottom three.
The visitors started brightly, Stephane Sessegnon fizzing an effort narrowly over in the second minute, before Allan McGregor parried away Graham Dorrans' effort 16 minutes later.
David Meyler wasted Hull's first real chance in the 27th minute, somehow directing a free header wide from six yards after Tom Huddlestone's superb delivery from a left-wing corner.
The striker, going away from goal, nipped in front of Dawson and then appeared to trail his leg in a bid to manufacture some contact, with Foy taking his time before awarding the spot-kick.
Foster then tipped Huddlestone's rasping drive over, but Long doubled the advantage from the resulting corner, turning home Davies' header after West Brom failed to properly clear.
Jelavic fired just wide six minutes after the break, and Rosenior cleared Victor Anichebe's header off the line three minutes later.
West Brom had strong penalty appeals waved away when James Morrison went down under the challenge of Jake Livermore, before Long's eventful day came to a premature end 12 minutes from time as he hobbled off injured after chasing a ball into the corner.
