The Tigers, who have already won the NatWest Trophy to deprive the Kings of one of their titles, only fleetingly overturned the two-point first-leg deficit.
With just over seven minutes remaining, Kevin St Kitts sank a long-range three-pointer to give Thames Valley the lead, on the night and overall, at 62-59. But Guildford's response was immediate, Lorenzo Duncan putting them back in front.
The Tigers never led overall again but stayed close enough to Kingston for Michael Obaseki to make it 74-72 with a thundering dunk 66 seconds from the end, scoring the final basket of regular time and forcing the extra period.
Guildford took the game by the throat with Duncan scoring the first six of their points in the extra period, and once Guildford had opened an 82-77 lead there was no way back for Thames Valley.
Guildford will play Worthing in the final after the Bears defeated Leicester 90-77 for an 190-176 aggregate victory.
LEADING SCORERS: Thames Valley Tigers: Lloyd, 20, St Kitts 20, Scott 17. Guildford Kings: Duncan 26, Henlan 18, Gordon 17.Reuse content