The Frenchman beat American qualifier Nishesh Basavareddy 7-6 (7/5), 6-4 in their semi-final to become the second-oldest men's singles finalist at any event since the ATP Tour began in 1990.
Monfils capitalised on opportunities better than his 19-year-old opponent, converting his only break point of the match late in the second set.
He said he was relieved to reach a 35th career final to kick-start his 21st year on tour, hoping it sets him up well for the Australian Open in Melbourne starting Sunday.
"I'm very happy. This kid has a bright future in front of him, he pushed me to the limit," said Monfils, who faces fellow Frenchman Giovanni Mpetshi Perricard in the first round of the Australian Open.
Saturday's Auckland final will pit the unseeded world number 52 against either seventh-seeded Portuguese Nuno Borges or unseeded Belgian Zizou Bergs.
