Downing struck his sixth goal of the campaign after 25 minutes and fit-again Heskey wrapped up the points 10 minutes from time before a late consolation from Paul Scharner.
Victory on Saturday ended a run of three successive Premier League defeats and eased Villa's relegation fears.
It was also a boost for Houllier, who had come in for criticism from Villa fans for his behaviour when returning to Liverpool for Monday's clash.
He was warmly applauded by the Holte End when making his way to the dug-out and then saw Downing and Heskey ensure Villa remain unbeaten against Albion since 1985.
West Brom were the better side in the first half with wingers Jerome Thomas and Somen Tchoyi seeing plenty of the ball.
Villa looked nervy but they were transformed in the second period with stand-in skipper Ashley Young a constant threat.
Heskey also caused problems in his first appearance after a seven-game absence following a knee setback.
Houllier made five changes - three of them enforced - including the axing of acting skipper Richard Dunne.
Albion, bidding for a hat-trick of league wins, were the first to settle.
A corner from Chris Brunt found its way to Marc-Antoine Fortune - called up in place of the injured Peter Odemwingie - at the far post but his low shot on the turn did not extend Brad Friedel.
The recalled Carlos Cuellar blocked a near-post Scharner header from another Brunt corner.
A fine pass from Graham Dorrans enabled Brunt to get in a shot, despite the attention of two defenders, which flew across the face of goal.
Villa hit back and Heskey got in front of Gabriel Tamas in connecting with a Young free-kick, but he glanced his header well wide.
Albion stepped up a gear and the woodwork denied Tchoyi the opening goal after 21 minutes.
He twice twisted past James Collins before beating Friedel with a shot which thumped against the crossbar.
Fortune should also have done better when he got free in the area but drove his shot wide of the far post.
It was against the run of play when Downing put Villa ahead. Dorrans lost possession deep in his own half and allowed Marc Albrighton to run to the byline.
His cross was flicked on by Young at the far post and then Downing got in front of Gonzalo Jara to turn the ball past Scott Carson.
Heskey had a shot blocked by Tamas after a lapse of concentration by Scharner.
Albrighton came close to doubling Villa's lead after 55 minutes. Downing supplied a teasing cross from the left which picked out Albrighton but he headed just wide when under pressure from Marek Cech.
Albion responded and a miskick by Jara fell invitingly to James Morrison 10 yards out, but Cuellar got in an important challenge and Morrison's shot drifted wide.
Thomas became the first player to be yellow-carded after an hour for a challenge from behind on Young.
Jara followed him into Mark Halsey's notebook three minutes later for another mistimed tackle on Young.
Heskey just failed to make contact with a low cross from Young but
with 10 minutes left the former England forward made sure of the points for Villa.
Albrighton's cross flicked off Collins across the face of goal and Heskey was perfectly placed to nod home from close range.
Scharner reduced the arrears with a minute remaining with a powerful close-range header and in injury time Thomas lifted the ball over Friedel - but also over the crossbar as Houllier's men secured maximum points.