Defender Mark Peters' heroics as an emergency keeper could not help Cambridge as they lost to St Albans.
Peters took over from Paul Crichton after he was sent off for a professional foul on Saints' striker Elliot Benyon after 55 minutes.
Tom Davies scored the resulting spot kick for the visitors' second goal.
St Albans had earlier taken the lead in the 22nd minute through midfielder Ram
Marwa, who made no mistake from 12 yards after collecting Benyon'a pass.
Cambridge Utd: Crichton, Bloomer, Peters, Morrison, Brown, Brady (Robbie Simpson 69), Bridges (Hanlon 25), Wolleaston, Carey-Bertram, Richardson (Marum 70), Josh Simpson.
Subs Not Used: Duncan, Quinton.
Sent Off: Crichton (55).
Booked: Bridges, Carey-Bertram, Josh Simpson.
St Albans: Bastock, Seeby, Elphick, Theobald, Damien Batt, Hann, Marwa (Okuonghae 90), Davis, Walshe (Flynn 90), Clarke, Benyon.
Subs Not Used: Perks, Cracknell, Simon Martin.
Goals: Marwa 22, Davis 56 pen.
Ref: A Rayner (Leicestershire).