The 24-year-old batsman fired a competition-best 69 not out as Sussex sealed their first appearance in a major final for 13 years on Friday.
Hopkinson and Mike Yardy (41) shared a crucial fifth-wicket stand of 95 in 19 overs as Sussex, set 255 to win, got home with seven balls to spare at Hove.
Sussex skipper Chris Adams paid tribute to his side after setting up their first Lord's final since 1993.
"I can't give enough credit to the youngsters in the side. It's going to be a great day out for us so bring on whoever," said Adams.
Sussex, who qualify for the final without having to win their last Southern Conference game against Gloucestershire, started strongly with Matt Prior (54) sharing an opening stand of 95 with Richard Montgomerie (44) in 16 overs.
But the hosts later slipped to 120 for four with skipper Chris Adams, Murray Goodwin and Montgomerie all falling in the space of five overs.
Hampshire, who chose to bat, were bowled out for 254 with three balls of their innings remaining.
Michael Carberry and Sean Ervine had given Hampshire a solid start by sharing in a second-wicket stand of 106 in only a dozen overs.
Hampshire, who needed to win to have any chance of reaching the final, lost Dominic Thornely and Greg Lamb cheaply before Dimitri Mascarenhas (42) and Nic Pothas (43no) took them past the 250 mark by sharing a sixth-wicket stand of 73 in 12 overs.
But the innings ended tamely with Kirtley bowling Shane Warne, Shaun Udal and James Tomlinson for ducks in his final over.
In the North Conference, Michael Vaughan took another step on the road to recovery but was unable to prevent Yorkshire slipping to a four-wicket defeat against Leicestershire at Grace Road.
The England captain, in his fourth game back following a recurrence of a knee injury, scored 36 off 66 balls as Yorkshire posted a total of 185 for seven.
But a half-century from Tom New and an unbeaten 46 by Paul Nixon eased the home side to their third victory of the season with 8.2 overs to spare.