United paid the ultimate cost for a poor first-half display and errors in both penalty-areas, despite an encouraging second-half display.
Faced with a team as young and inexperienced as their own the Blades opened sluggishly and the visitors dominated the first period with a quick, incisive and accurate passing game whilst United failed to impose their more direct approach.
Visiting striker Jennings had already put a free header far too close to goalkeeper Billard by the time Tranmere did take the lead after 10 minutes.
Gibbs split the defence with a splendid through ball which MACKRETH ran on to and calmly slotted under the exposed goalkeeper.
Billard was frequently called upon to race out and kick clear as the speedy visiting forwards surged through on the counter-attack, and it was no surprise when the lead was doubled in the 23rd minute.
Fortune misread a high ball and instead of cutting it out allowed MACKRETH another opportunity to run on and score, this time from the left.
Four minutes later the Blades at last created a good chance but Slew miscued Brown's cross when he should have at least hit the target from close-range.
It at least signalled a spell of pressure in which Foster pulled a shot wide, Williams drove at the goalkeeper and Foster headed Roe's centre too close to Collister.
United needed to take advantage of this period of play and in the 40th minute managed to reduce the arrears. Williams slipped as he played the ball forward but it reached its target and FOSTER did well to retain his balance as he angled his shot past Collister into the empty net.
But slackness in defence just a minute later saw JENNINGS glance a header past Billard to restore the two goal cushion going into the break.
Two substitutions stiffened the Blades line-up and they took the game to Tranmere with much more purpose and effect, pulling the score back to 3-2 six minutes into the half'
James, now in midfield, was fouled and ROE curled a delightful free-kick into the top corner from 25 yards.
The full-back was just off target shortly afterwards from an identical position, and Slew was then on target but found the goalkeeper.
Pennycooke-Morgan was proving a lively substitute and got into a shooting position but couldn't beat Collister at his near post.
The Blades were definitely in the ascendancy but just past the hour Slew wasted a real chance, skying over after great work from Pennycooke-Morgan.
Another opportunity of equalising went begging as the unfortunate Slew, after neat footwork which set him up, struck Collister's legs.
It was looking just a matter of time before United scored again and they had really pulled themselves together to push the visitors back.
Foster and James both saw shots blocked by desperate defending in one frantic melee, and Slew hit the target but saw his shot comfortably saved.
With Fortune thrown forward in the dying minutes Tranmere broke and won a penalty when Brown felled Mackreth.
But the winger failed to register a hat-trick because Billard guessed correctly and plunged to his left to save the spot-kick.
United: Billard; Brown, Roe, Wilkinson (O'Kane h-t), Fortune, James, Murray (Pennycooke-Morgan h-t), Williams, Foster, Slew, S-Latef (Gregory 68).
Unused: Andrew, Lammy.
Tranmere: Collister; Pursehouse, Benson, Baker, Ashcroft, Roberts, Mackreth, Power, Jennings, Gibbs, Fraughan (Oliver 68).
Unused: Rule, Coughlin, McNamara, Fairclough.
