Chris Hughton's men held firm in the face of near incessant attacking from the pre-season title favourites, with Australia goalkeeper Ryan helping keep out City until the 70th minute, but a late strike from Aguero and Lewis Dunk's own goal sealed victory for Pep Guardiola's side.
Guardiola paired Gabriel Jesus and Aguero in attack, with new arrivals Kyle Walker and Danilo providing support from the flanks in a 3-5-2 system.
City dominated possession and territory, but the extra defender perhaps took something out of their attack and Ryan's stunning save from Jesus was City's sole clear-cut opportunity of a goalless first half.
Jesus also saw two goals disallowed as wave after wave of City attacks broke up on Brighton's back line, until Dale Stephens's error opened up space for a merciless counter-attack and City's opening goal and Dunk's unfortunate intervention settled the match as a contest.
Having overseen the first trophyless campaign of his coaching career last term, Guardiola cut a frustrated figure for much of the game, but Chelsea's defeat to Burnley and Liverpool drawing at Watford earlier in the day paint this below-par victory in much kinder light.
The party atmosphere at the Amex Stadium never suffered, though Brighton registered just a single shot in an opening half that saw one of the entertainers of last season's Championship season display a steelier side.
There were 21 players in Brighton's half as early as the third minute and Aguero lashed fresh air from Walker's cut-back early on, before Kevin De Bruyne's set-piece sighter gave Ryan a comfortable first Premier League stop.
De Bruyne's probing almost paid off in the 27th minute when a measured ball over Brighton's defence was bundled in by Jesus, but referee Michael Olivier spotted the Brazil striker's hand making the crucial final touch and the goal was ruled out.
City were soon back in their stride, though, and only a razor-sharp Ryan save denied Jesus's planted header before the off-balance striker thrashed the rebound against the post.
Brighton counter-attacks regularly broke up against City's three-man defence, but the half-time whistle was greeted with loud cheers regardless after 45 minutes of stiff resistance.
Jesus saw another goal chalked off shortly after the restart, with David Silva straying fractionally offside on the end of a stunning Fernandinho pass and Aguero spurned a one-on-one opening under pressure from Dunk as City resumed their dominance of the contest.
Brighton's first spell of concerted pressure followed and Shane Duffy's shot after a corner-kick melee was deflected wide before Ederson made a hash of the following delivery and Davy Propper drilled narrowly off-target.
But the hosts were delivered a lesson in the Premier League's ruthlessness with 20 minutes to go as City finally went ahead.
Stephens lost possession on halfway and City raced through an unset defence with De Bruyne finding Silva, whose instant pass into Aguero allowed the master marksman to take a touch and finish.
Five minutes later, Dunk thundered a header past Ryan in challenging Jesus to put the result to bed, but the centre-back's place on the scoresheet was unjust after a fine display.
Key Opta stats:
- Manchester City have won each of their last seven opening day contests in the Premier League, the best current run in the competition.
- Brighton have won just one of their last six opening fixtures to a league season (D1 L4).
- Sergio Aguero has now scored against 30 of the 31 sides he has faced in the Premier League (only failing to score against Bolton Wanderers).
- Since his Premier League debut in August 2010, David Silva has provided 65 assists in the competition, 14 more than any other player (Wayne Rooney next on 51).
- Goalkeeper Mat Ryan attempted more passes than any outfield Brighton player in this match (27).