It was the seventh win in eight games for last year's runners-up but they were fortunate against feisty opponents as they stayed top of their conference standings.
The teams traded converted tries to be level with 10 minutes to go but the Lions won a penalty two minutes from time which Jantjies slotted over to seal victory.
The Jaguares struck inside the opening five minutes to serve notice of their determination to halt a run of two successive losses as Matias Orlando burst over to score after a break by influential No 10 Nicolas Sanchez.
The strength of the Lions enabled a quick recovery, however, and Harold Vorster scored the first of their two first-half tries 11 minutes into the game.
The Lions then set up a chance for centre Lionel Mapoe, who danced through the Argentine defence to score after a clever show-and-go.
It gave the hosts a seven-point lead at the break but they were quickly 21-14 down as the Jaguares dominated the first 20 minutes after the break.
Loose forward Rodrigo Baez stretched over in the 48th minute to level the scores and three minutes later it was lock Guido Petti out wide on the wing who crashed over after the South Americans went the length of the field.
The error-prone Lions missed several chances to pull level but with 10 minutes left Ruan Ackermann finished off an attacking move that started deep in the hosts own half before Jantjies' coup de grace.
Photo By: GALLO IMAGES (Harold Vorster of the Lions passes the ball as the Jaguares attack).