Errani dropped the first set but rebounded with a second-set tiebreaker and went on to outlast Pennetta 5-7, 7-6 (7-2), 6-0, in a 2-hour, 28-minute contest at the Fairmont Acapulco Princess.
Errani dominated the tiebreaker 7-2 to take the second set, then turned it on, winning 10-of-13 first serve points and 12-of-15 second return points in the final frame.
Errani logged her third career triumph in the first all-Italian WTA final since Barcelona in April 2010.
Pennetta, a two-time Mexican Open winner (2005 and 2008), was also the runner-up in Acapulco in 2005, 2006, 2007 and 2009.
She fell to 9-13 in career in title matches.