Marbury scored 11 of his 26 points in the second period, and Marion 18 of his 27 in the third as the Suns cruised.
It was the biggest loss of the season for the Pacific Division-leading Kings, who missed an opportunity to move to within 2 1/2 games of Dallas for the top record in the Western Conference. The Mavericks lost at Seattle.
"We just played awful," said Kings Coach Rick Adelman. "That's all I have to say. We didn't have any energy or urgency to play. If we play like that, we're going to have a hard time down the stretch."
Marbury's second-quarter production turned a 20-19 edge into a 45-29 lead with 2:41 left before Sacramento climbed to within 49-42 at the break.
Marion then nailed three three-pointers in the third period, and Marbury closed the quarter with a rare four-point play to make it 80-67.
The Kings got no closer in the final 12 minutes.
"We got it going," Marion said. "We stuck with it and we just opened up everything. It got everybody going. Everybody was involved. We were just doing what we wanted to do out on the court."
"Tonight, this was the Shawn we're used to seeing," said Suns Coach Frank Johnson. "He was very efficient, taking shots and making passes."
Joe Johnson and reserve Jake Voskuhl added 13 points apiece for Phoenix, which had lost nine of its first 13 games after the All-Star break.
With the win, their biggest of the season, the Suns (34-30) moved one-half game ahead of idle Houston for the eighth and final Western Conference playoff berth.
"We needed it," Marbury said. "Against the good teams, we know we've got to play. We have to take the same approach when we go down to Houston (on Sunday)."
Chris Webber collected 22 points, 11 rebounds, and nine assists, and Mike Bibby added 20 points for the Kings, who shot just 37 percent (33-of-90) from the field, and allowed the Suns to connect on 49 percent (38-of-77).
"I think you have to look in the mirror," Webber said. "I think they played well, but I kind of question our effort tonight. We need to make sure that we come back and prove that we are the team we think we are."
Sacramento, which had won nine of 10 overall and eight of 11 over Phoenix, was missing starting center Vlade Divac, who is day-to-day with bronchitis.
Phoenix outrebounded Sacramento, 51-40. Besides Marion, rookie Amare Stoudemire (11) and Bo Outlaw (10) had double-digit rebounding performances.