It was a good day for the Pilgrims as their play-off rivals Newport County, Luton Town and Exeter City all lost.
Argyle are now two points ahead of eighth-placed Newport and three in front of Luton and Exeter in ninth and 10th respectively.
It took John Sheridan’s side less than 100 seconds to take the lead in front 6,626 fans at Home Park when fit-again Anthony O’Connor headed in a Bobby Reid free-kick. O’Connor had missed Argyle’s last two matches with a groin injury.
After the interval Luke McCormick was forced to make a good save before the Greens, without suspended captain Curtis Nelson, went 2-0 up from another set piece. Carl McHugh rose to head in a Bobby Reid corner, although O’Connor has claimed to have got a final touch on the ball before it went into the net.
Mansfield, who arrived in Devon on the back of four straight defeats, hit back with a goal by Vadaine Oliver. But Argyle were able to hold on and claim three vital points.
They are in action again at home on Tuesday when they entertain AFC Wimbledon.