The game got off to a cagey start, the first chance coming on eight minutes when Lee Evans fired just wide of the Lions' post from outside the area.
Both sides traded blows throughout the first half, Blades defender Richard Stearman forcing a smart save from Archer just before the whistle, but neither could break the deadlock by half time.
The breakthrough came on 74 minutes when Noel Clarke headed home from close range following a delivery across the face of goal.
The latter stages of the game saw both teams frantically press for a winner, but both goalkeepers were on form as the final whistle heralded a share of the spoils in South Yorkshire.
The draw moves Millwall up to fifth in the table, while the Blades are eighth, two points outside the playoffs.
SHEFFIELD UNITED (3-5-1-1): Blackman; Basham, Stearman (Lafferty 70'), O'Connell; Baldock, Lundstram (Duffy 45'), Evans, Fleck, Stevens; Brooks (Sharp 59'), Clarke
MILLWALL (4-4-2): Archer; Romeo, Hutchinson, Cooper, Meredith; Wallace, Williams, Saville, Marshall (Onyedinma 84'); Gregory (Elliott 90'), Morison