The 30-year-old, who completed a £51million move from Aston Villa last week, opened his account for his new club four minutes from time, heading home a cross from Sultan Mandash at the back post.
Introduced by manager Simone Inzaghi at half-time in place of Mohamed Kader Meite, Watkins came off the bench to add a third to goals from Sergej Milinkovic-Savic and Crysencio Summerville in a comfortable win at the Kingdom Arena.
The victory kept Al-Hilal top of the Saudi Pro League after four games without dropping a point. Speaking after the match, Watkins said he had been struck by the standard of Saudi football.
"It was a very good start. The level I saw here today was honestly of Premier League quality," he said.
"The match was very strong physically and demanding, and the quality was extremely high."
Watkins arrives in Riyadh at the peak of his career rather than as a late-career money move, following six seasons at Aston Villa in which he became the club's all-time leading Premier League goalscorer. He joined from Brentford in 2020 and helped Unai Emery's side win the Europa League last season, prompting an emotional send-off from his manager, who said he had "small tears" saying goodbye to a player he called "fantastic."
Watkins is the latest in a string of Villa exits this summer, following Morgan Rogers, Ezri Konsa, Youri Tielemans, Lucas Digne and Emiliano Martinez out of the exit door at Villa Park. He joins fellow Premier League export Crysencio Summerville, formerly of West Ham, in Al-Hilal's ranks.
The transfer also carries a financial knock-on effect back in England. Exeter City, where Watkins began his career, say they expect to receive a payment through FIFA's solidarity mechanism, which distributes a share of transfer fees to clubs involved in training players between the ages of 12 and 23.
The League Two club said any windfall would be used to "protect and strengthen the long-term security of the club."
Watkins becomes the first English player to appear for Al-Hilal, a landmark that added extra significance to an evening in which the visitors were reduced to 10 men before half-time. Al-Ahli, still seeking a first win over their rivals since surrendering a three-goal lead against them last season, offered little resistance once Watkins was introduced, with Inzaghi's side comfortable throughout the closing stages.
For Aston Villa, the sale represents a substantial financial return on the £28million paid to sign Watkins from Brentford, even as Emery works to bed in Chelsea striker Nicolas Jackson as his replacement. Watkins scored 108 goals in 280 appearances during his time at Villa Park, a tally that stood as the club's Premier League record before his departure for the Middle East.