Matt Sparrow and debutant Sam Winnall were both on target for the Iron in the first half, but it was the Stags, back in the Football League after a five-year absence, who made the livelier start.
They could have been ahead in the second minute, but striker Lee Stevenson wasted a good chance in front of goal when he dragged his shot wide.
Scunthorpe, with six summer signings in their starting line-up, took longer to settle.
But when they did, they took control with two goals in 10 minutes.
They opened the scoring in the 29th minute with what was their first threat of the match.
A left-wing cross from Andy Welsh found midfield man Matt Sparrow lurking unmarked at the far post and he celebrated his return for a second spell with his hometown club by rifling a right-footed shot into the top corner.
Marriott then twice kept Mansfield in the game with good saves from full-back Andy Dawson and Winnall before the home side struck again in the 39th minute.
The Mansfield defence was opened up by an excellent through-ball from Terry Hawkridge and ex-Wolves man Winnall celebrated his debut for Scunthorpe by scoring at the second attempt.
Mansfield missed a chance to get back into the game in the 70th minute when their finshing again let them down as Ross Dyer headed wide from six yards.
Their misery was then compounded in the final minute when substitute Louis Briscoe missed from the penalty spot after a handling offence which saw Scunthorpe full-back Eddie Nolan sent off for a second bookable offence.