Chelsea's club-record signing had failed to score in five games before arriving in Abu Dhabi, and the Blues are looking for the 28-year-old to find his best form in the second half of the season.
The Belgium international swept in from close range on 33 minutes after a defensive lapse to send Chelsea through to Saturday's final, the second in three years between English and Brazilian teams.
Chelsea can join Manchester United and Liverpool as the third English side to win the Club World Cup, having finished as runners-up to Corinthians on their first appearance in 2012.
They may yet have coach Thomas Tuchel in the dugout come the weekend, with the German still isolating back home after testing positive for Covid-19.
Al Hilal, coached by former Monaco boss Leonardo Jardim, will contest the third-place play-off against Egypt's Al Ahly as Chelsea's win made it 12 from 12 for Europe against Asian clubs in the tournament.
Despite the presence of former Manchester United and Watford forward Odion Ighalo alongside ex-Porto striker Moussa Marega in attack, Al Hilal struggled initially to test Kepa Arrizabalaga in goal.
The Spaniard kept his place in the line-up despite the return of Edouard Mendy following his Africa Cup of Nations triumph with Senegal.
Hakim Ziyech fired a shot narrowly over and Lukaku failed to make proper contact with Cesar Azpilicueta's low ball across goal, but there was no shortage of service for the Belgian.
He held off a defender only to strike straight at Abdullah Al-Mayouf, but Lukaku was presented with the simplest of chances when Yasir Al-Shahrani's failed clearance left him to turn in the opener.
Kai Havertz, whose cross led to Lukaku's goal, clipped an effort against the outside of the post after jinking into the area early in the second half.
Ziyech then drew a good stop from Al-Mayouf before Al Hilal, who put six past Al Jazira in the previous round, belatedly sparked into life around the hour.
Arrizabalaga raced off his line to block from Marega and then pulled off a sensational one-handed save, flying to his left to keep out Mohamed Kanno's curling attempt from 20 metres.
Matheus Pereira tormented Chelsea on his last outing against them while at West Brom last season, but the Brazilian's fizzing effort into the side-netting was the last glimmer of hope for the record four-time Asian Champions League winners.