Michael Walton, Javier Quevedo and Pablo del Olmo share second place at minus-6.
Eric Wang and Chris Wall are knotted in fifth place at 5-under-par 67.
Miller began on the back nine Thursday at El Cid and rattled off three consecutive birdies from the 11th. He added another at the 17th to make the turn at 4 under par.
He continued his strong play on the second nine as he birdied the par-5 second and par-4 fifth. Miller eagled the par-5 seventh to reach 8 under par, but a bogey at the last dropped his lead to one stroke.
Walton, playing his second season on the Canadian Tour, collected seven birdies and a bogey. He tied for second in his last visit to Mexico at the Morelia Classic last year.
Quevedo also tallied seven birdies and a bogey, while del Olmo mixed an eagle, seven birdies and three bogeys for his 66. Del Olmo, who played on Mexico's World Cup team, won the Canadian Tour's first visit to Mexico four years ago as a sponsor's exemption.
Brian Flugstad, Jay Reynolds, Craig Taylor and Anthony Rodriguez are tied for seventh place at 4-under-par 68.