Chris Jordan had Adam Ball caught for a duck as the hosts were reduced to 2-1.
But Dickson (81) and Denly (78) shared a 162-run second-wicket partnership before the former was caught at backward point off Stu Whittingham.
Whittingham dismissed Denly after tea but Northeast's unbeaten 75 saw the home side close on 310-3.
Sean Dickson, Joe Denly and Sam Northeast hit half-centuries as Kent's batsmen dominated against Sussex on day one at Tunbridge Wells.