Prolific non-league Lee Stevenson had bagged his first ever Football League goal in the 22nd minute to put the home side ahead.
But, although Rovers' Steve Mildenhall pulled off the save of the match to deny Matt Rhead, much of the game saw Rovers in possession and Mansfield on the back foot.
Rovers played the better football in the first half and created more chances, but found themselves going in a goal down at the break.
Home keeper Marriott was forced into saves from Lee Crown at his near post from a tight angle and then from the edge of the box from Ollie Clarke and David Clarkson.
A missed clearance from Calvin Andrew also offered Clarkson a chance which he fired wide of the far post.
But the moment that counted came when Andrew launched a fine cross in from the right touchline which saw Stevenson jump to plant a well-placed header past Mildenhall.
Rovers again bossed long spells of second half possession but lacked penetration against the well-drilled home back four. Andy Bond had a shot from distance saved while Ollie Clarke shot wide from 18 yards.
Another Rovers break saw home defender Martin Riley halt Eliot Richards and then block Ryan Brunt's follow-up shot.
The Stags might have doubled their lead in the 66th minute as Mildenhall pulled off a great reaction save to keep out Rhead's goalbound header from a Sam Clucas corner.
But Rovers grabbed their deserved equaliser in the 84th minute. Marriott and Murtagh both went for a low Alefe Santos cross into the near post and the defender accidentally lifted the ball over his keeper's dive into the net.