Fernando Torres had already hit the post when Rodallega made the most of poor defending from the Reds to fire the hosts in front just after the half hour.
It was Wigan's second win of 2010 and their first in eight matches which sees Roberto Martinez's side move four points clear of the relegation zone.
Having beaten Chelsea earlier this season, it was also only the second time Wigan have ever beaten a "big four" team.
But Argentina captain Mascherano did not seem to mind the unfamiliar role and quickly got forward to good effect.
Liverpool began strongly and produced a threat again when Fernando Torres' volley flew wide off the post from Yossi Benayoun's deep cross.
Titus Bramble was forced into a fine block after Gerrard broke through midfield again and tried his uck from long range.
But struggling Wigan recovered from their early hesitancy and caused Liverpool plenty of problems.
The Reds produced a fine passing move with Maxi Rodriguez and Benayoun before Gerrard scuffed his shot wide from close range.
But Wigan made no mistake when Dirk Kuyt gave the ball away on the edge of his own area.
Emmerson Boyce curled a cross over and Rodallega fired in from close range to give the strugglers a shock lead.
And Torres failed to level when he headed over from six yards after he was picked out by a Rodriguez cross.
And as Torres tried to get Liverpool on level terms, he fired over from a low Rodriguez cross.
Benitez had finally seen enough of Lucas and dragged the Brazilian off to replace him with Johnson.
And Bolivian forward Moreno forced a fine save from Reina with a powerful drive after he cut in from the right.
N'Zogbia nearly created a second when his low shot beat Reina and Paul Scharner failed to touch in at the far post.
Liverpool pressed and pressed but failed to trouble Latics goalkeeper Chris Kirkland.
Substitute Victor Moses could have made the game safe for Wigan but shot narrowly wide as the hosts pressed hard on the break in added time.
They also survived a late goal-mouth scramble to grind out a first-ever victory over Liverpool and leave major doubts about whether the Reds can reach the Champions League.
Source: AFP Global Edition