Dutch trialist Brian Tevreden, from FC Volendam played on the left of midfield and Kevin Blackwell must have seen enough after only 30 minutes when he substituted him with Danny Pugh.
Scott Carson was in good form as he denied Town a few scoring opportunities. United had their first chance on the half hour when Brian Deane had a shot blocked on the line from the edge of the area with strike partner Michael Ricketts following in but Mansfield cleared the danger.
The Whites nearly took the lead five minutes before half-time when Matthew Spring started a move to Frazer Richardson who delivered a low cross straight to Rickett's feet inside the area but he could only slice his shot well wide.
Blackwell made no changes at half-time and then on 52 minutes Deane appeared to be tripped as he went to meet a Pugh cross , but the referee gave nothing.
United made a double substitution on 62 minutes with Ricketts and Spring making way for Julian Joachim and another trialist, Mark Kinsella respectively.
Then, out of the blue on 72 minutes, Leeds went behind. Mansfield winger Wayne Corden picked the ball up from 25-yards and, with a speculative shot, gave Carson no chance as it went into the top right hand corner.
Four minutes later it was 2-0 when Wolves striker Colin Larkin surprised everyone with a fine chip from the edge of the area over Carson.
An early wake-up call for Leeds and the board, who must surely start putting their hands in their pockets for some quality players for the season ahead.
Mansfield: Pilkington, McNiven (Buxton 79), Eaton, Artell, John-Baptiste, Neil (Dimech 79), Williamson, Murray (McKenzie 57), Asamoah (Herron 71), Robinson (Larkin h/t), Corden.
Leeds: Carson, Kelly, Duberry, Butler, Kilgallon, Richardson (Cadamarteri 73), Spring, Wright (Hignett 81), Tevreden (Pugh 30 (Guppy 76)), Deane, Ricketts. Unused sub: Carlisle.
Attendance: 5,294