Two goals from Billy Sharp helped the Blades to victory on the east coast.
Dominic Calvert-Lewin was also on target as United completed the fourth of six pre-season fixtures.
Long started his first game of pre-season and was barely involved early on as United began on the front foot. Sharp's impressive control and turn from Hussey's cross resulted in a shot just over the bar and then Duffy curled just wide from 25 yards.
United's early dominance was then rewarded with two goals inside three minutes, with the two strikers very prominent.
On 23 minutes CALVERT-LEWIN took a controlling touch before lifting over goalkeeper McKeown to convert Sharp's deft cross. Then, on 25 minutes, Calvert-Lewin won possession, laid off for Done and although his shot was blocked on the line by Gowling, SHARP nodded the rebound into the corner of the net.
Efforts from Pearson and Berrett failed to get the Mariners back into the game and United went close to extending the lead. McKeown saved from both Brayford and Done, the latter effort following fine build up place from Sharp.
The Blades also started the second half well. Wilson had an effort cleared off the line and Duffy had appeals for a penalty turned down but the home side got back in the game with a goal out of the blue on 54 minutes, an inswinger from SUMMERFIELD beating Long.
Further appeals for a penalty, this time Davies and Sharp clashing, were ignored and then a marauding run from Brayford set up an opportunity for Sharp to crash an effort against the crossbar.
The United captain, though, did grab a second with his last touch before being substituted. Duffy put SHARP in the clear and although his first effort was blocked he kept his cool to slot home at the second attempt in front of the Blades fans.
In the later stages, as more substitutions began to take effect, United continued to control the game. O'Connell clipped over a Hussey cross, a Basham shot was deflected and McKeown was quickly off his line to stifle Adams.
Blades: Long, Brayford (Freeman 86), Hussey (K Wallace 86), Wilson, O'Connell, Fleck (Brooks 78), Basham (Whiteman 82), Done (Scougall 67), Duffy (Coutts 67), Calvert-Lewin (McNulty 45), Sharp (Adams 61). Subs: Ramsdale, Semple, Wright.