The striker opened the scoring when he collected a superb long ball by goalkeeper Ederson and chipped in.
City were 3-0 up by the 35th minute as Gabriel Jesus netted a low drive and Aguero capitalised on a Ben Hamer error.
Huddersfield's Jon Gorenc Stankovic pulled one back before David Silva marked his 250th Premier League appearance with a free-kick into the top corner.
Aguero sealed his ninth PL hat-trick, flicking in Benjamin Mendy's third assist of the season, while substitute Leroy Sane forced a late own goal from Terence Kongolo.
Man City: 25 Aug v Wolves (A)
Huddersfield: 25 Aug v Cardiff (H)