The Blades have been confirmed as champions as EFL Sky Bet League One.
Bolton's failure to beat Oldham on Saturday means none of the other 23 teams in the division can overhaul United's points tally of 91.
United are just the fourth team to win all four top tiers of English football - Wolves, Burnley and Preston are the others.
United's title wins:
1st tier - 1897-98
2nd tier - 1952-53
3rd tier - 2016-17
4th tier - 1981-82
United will be presented the Sky Bet League One trophy at the final game of the season, on 30th April, when Chris Wilder's side face Chesterfield.