The British showjumper also took second place on his 11-year-old stallion Leonitas, with the Mexican Nicolas Pizarro finishing third with Saskia.
Silver Exchange II – an 11-year-old Oldenburg gelding by Lavallo – completed the jump-off with a time of 43.295 seconds.
McPherson said: “My tactics were really simple. I just wanted to jump clean rounds with these two.
“Thank God all my dressage is paying off somewhere down the line.”
McPherson has ridden Silver Exchange II for just over a year.
“[He] finished off last year wonderfully [and] then he was on a long layoff.
“I’ve come here and just jumped last weekend and this weekend [and] so I’m delighted with him.”
McPherson and Leonitas also received third place in the $8,000 G&C Farm 1.45m jumpers class earlier in the week.
The 2012 FTI Consulting Winter Equestrian Festival runs over 12 weeks, finishing on 1 April. More than $6m ( £3m) of prize money will be awarded through the circuit.