Maguire, 25, and Ellen Whitaker, 19, captured the top two places, with the veteran Frenchman Jean-Marc Nicolas third.
Ireland's Billy Twomey and Britain's Geoff Luckett completed weekend doubles. Twomey's victories (which included yesterday's Speed Horse of the Year title) were achieved with a borrowed mare, Intermission, which he had sat on for the first time on Thursday.
The atmosphere of bygone Horse of the Year Shows had been recaptured in a wonderful finale to the Puissance on Saturday night when John Whitaker, a crowd favourite, cleared the wall at 7ft 1in on both his mounts Lactic II and Exploit du Roulard leaving the two horses as equal first.
Not to be outdone by his uncle, William Whitaker filled the top two places in yesterday's Kitt-Kraiburg Junior Show Jumper of the Year with Indien de Here and Colton Maelstrom. Another brother, Michael, was competing for the record prize fund of $1m (£635,000) on offer at the inaugural Las Vegas World Invitational.
Brazil's Olympic champion, Rodrigo Pessoa, took the plum reward of $250,000 when winning the Grand Prix, with Nick Skelton collecting second prize ($178,000) and Michael Whitaker fourth ($75,000).