The Mariners, who overcame Essex part-timers Braintree United in the semi-finals, are back in League Two for the first time since the 2009-10 season.
The 2010-11 campaign was Grimsby's first in non-League football after 117 years in the Football League, but after losing the playoff final last season, Paul Hurst's side are back in the fourth tier of professional football.
A double from 23-year-old striker Omar Bogle saw Grimsby take a 2-0 lead late in the first period, but Forest Green were back in the argument when Keanu Marsh-Brown found the back of the net with an effort from distance on the hour.
Cheltenham Town were also promoted to League Two after winning the National League. body check tags ::