En route to 14 points, Thompson shot 4 for 7 from beyond the arc. His back-to-back threes early in the first quarter provided a six-point cushion that the Prep, despite 11 second-half turnovers, never relinquished.
Roman had turned back visiting Prep, 62-58, on Feb. 3. "I didn't shoot too well at their place," Thompson said. "I remember missing my first three shots."
At 8:15 p.m. Monday, back at the Palestra, the Hawks will vie for top honors against the winner of Wednesday night's second semifinal, between Archbishop Carroll and Neumann-Goretti.
In last year's final, the Prep bowed to N-G, 59-57, in overtime.
"We're happy to make it back," Thompson said. "It'd be great for us to get over the hump and win it."
Against the Cahillites, an 8-0 run at the start of the second quarter gave the Hawks (20-4) a 25-11 advantage. The flurry consisted of Miles Overton's trey, Steve Vasturia's layup, and Overton's bomb from up top.
The Wake-Forest bound Overton totaled 17 points, four rebounds, four assists, and three steals. Vasturia, bound for Notre Dame, posted 16 points, eight boards, and four assists.
With 21/2 minutes left in the third quarter, Thompson's left-wing three gave the Prep its largest lead, 38-23.
"He's been shooting real well lately," Hawks coach Speedy Morris said.
Roman, with its full-court pressure, forced nine turnovers in the third quarter. In the fourth, Rashann London's pull-up jumper trimmed the gap to 51-48 with 1 minute, 9 seconds to play.
In the final minute, P.J. Kelly's steal of a crosscourt pass and clutch free-throw shooting from Vasturia (8 for 8 overall from the line) and Overton (5 for 6) sealed the deal.
Shep Garner paced the cold-shooting Cahillites (18-6) with 15 points, six boards, and three assists. TreVaughn Wilkerson managed 10 points and nine rebounds.
St. Joseph's Prep 17 13 12 15 – 57
Roman Catholic 11 5 15 19 – 50
SJP: P.J. Kelly 3, Steve Vasturia 16, Miles Overton 17, Chris Clover 7, Kyle Thompson 14.
RC: Rashann London 8, Shep Garner 15, Manny Taylor 5, Traci Carter 9, TreVaughn Wilkerson 10, Matt Simon 3.
Contact Rick O'Brien at firstname.lastname@example.org, or @ozoneinq on Twitter. Read his blog, "The O'Zone," at www.philly.com/ozone