Shane Long was left on the bench against his former club, but it needed the second half introduction of the three-goal striker to tee up the winner for Lukaku.
Victory enabled Albion to complete a trio of wins in their opening three home fixtures of a top flight campaign for the first time in 58 years.
Albion were the more threatening for long periods against Brian McDermott's side despite not firing on all cylinders.
James Morrison impressed in midfield and was the official man-of-the-match while Lukaku posed problems with his pace and power before delivering the killer touch.
It leaves Reading still searching for their first win since returning to the Premier League, and West Brom keeper Ben Foster was untroubled throughout the 90 minutes.
Albion started brightly and Reading defender Chris Gunter had to react quickly to cut out a Lukaku flick-on intended to pick out skipper Chris Brunt.
Lukaku skipped past the challenge of Kaspars Gorkss and drilled a dangerous low ball across the box but Danny Guthrie was alert to the situation.
Brunt produced the first serious attempt on goal with Royals keeper Alex McCarthy turning his low 25-yard drive around the post at full stretch.
Reading threatened when Garath McCleary attempted an overhead kick which contained plenty of power but flew straight at Foster.
Lukaku continued to pose problems and McCarthy finger-tipped away a floated cross at full stretch aimed at the run of Brunt at the far post.
Albion looked the more likely to score and James Morrison found space in the box after exchanging passes with Zoltan Gera but he scuffed his shot across the face of goal.
Morrison seemed in two minds whether to shoot or pass when he got in behind the Reading defence and his eventual chip was too high for Lukaku and summed up a disappointing opening half.
The nearest either side came to scoring in that period came at the end of the first half when Albion defender Jonas Olsson's header hit the angle of bar and post from a Brunt corner.
Albion continued on the offensive after the break with Gera having a shot charged down.
Baggies defender Liam Ridgewell vented his anger at Pavel Pogrebnyak after feeling the striker had gone down too easily under a challenge.
McCarthy had to be alert to push aside a fierce dipping volley from Morrison as West Brom looked the more likely to break the deadlock.
Former Albion defender Nicky Shorey floated a free-kick wide in his first start since rejoining Reading during the summer.
West Brom fans chanted Long's name before he was introduced after 65 minutes - and six minutes later he had played his part in the winner against a side he spent six seasons with.
Another substitute in Graham Dorrans floated a ball into the box which was knocked by back by Long to Lukaku who steadied himself before driving a low shot past the dive of McCarthy.
Reading tried to respond and substitute Hal Robson-Kanu peeled off at the far post and got on the end of a McCleary cross.
But his header landed on the roof of the net and the home side comfortably saw out time in the closing stages.