Albion's PL2 side couldn't make their dominance count as they were edged out by Middlesbrough on Friday afternoon.
Tyrone O'Neill's fine effort in the first half proved to be the deciding strike, but it didn't reflect the Baggies' pressure and chances throughout the game.
Owen Windsor, Kamil Grosicki and Saul Shotton all came desperately close to netting an equalising goal, but the visitors held out to depart the West Midlands with all three points.
Deon Burton was joined in the dugout by Albion legend Chris Brunt, who made his coaching debut against the club whose Academy he progressed through.
The Baggies side included senior figures in the starting XI for the second game in a row, as Kamil Grosicki and Kyle Edwards took up the wide positions.
And it was Edwards who made an early impression when he dribbled his way through the Boro midfield and into the box, only to see his close-range shot parried away by Boro' stopper Sol Brynn.
The visitors tried to utilise a set-piece routine as they fought for the opener, but Joseph Gibson's strike from the corner flew well over Ted Cann's crossbar.
Finley Thorndike responded to Boro's pressure well as he closed the visitors' defence down to win a bouncing ball in the box, but despite the favourable position, he could only hit the side netting as Burton's men piled on the pressure.
However, for all the home side's confidence and chance creation rate in the opening stages, it was Boro who took the lead.
Forward O'Neill capitalised on a loose ball before swivelling and guiding a powerful finish into the top corner to give the visitors the advantage against the run of play.
Albion created several glorious opportunities to equalise in the first half as Windsor and Harmon had chances just a few yards out from Boro's goal, but they couldn't force the ball in as the Baggies went in at the break trailing.
Windsor continued to influence proceedings after the interval and had an opening right at the start of the half which just evaded him as the forward nearly found himself through on goal after a through ball had worked out favourably for him. But the offside flag went up to rule out a potential equaliser for the hosts.
Boro maintained their resolve, but invited pressure on as Grosicki nearly scored a tremendous goal from distance as he curled an effort goalwards in the hope of catching Boro stopper Brynn out of sorts. But the keeper kept his concentration to tip the Poland international's shot over.
Baggies full-back Harmon tried his luck from range as he dragged an effort wide, and Thorndike and Grosicki both saw close-range efforts go over the bar.
And the hosts nearly netted a deserved equaliser in stoppage time when centre-back Saul Shotton was presented with a great chance after Ethan Ingram's long free-kick looped over the defence and into his path.
But Shotton couldn't guide his effort on target as the Baggies suffered defeat and were made to rue a number of missed opportunities.
Albion: Cann; Ingram, Shotton, Delaney (Sharpe 72), Harmon; Edwards, Brown, Iroegbunam (King 65), Grosicki; Thorndike; Windsor.
Substitutes: Boruc, Gardner-Hickman, Dyce.