Lee Philpott was sent off for a second bookable offence but United, who had held the initiative throughout, extended their lead before County grabbed a late consolation.
Cambridge, whose manager John Beck was sacked last week, took a 10th-minute lead with Gary Clayton and they held it until the 51st minute when Mark Draper's shot took a deflection off Danny O'Shea. Mick Danzey, on his debut, chested home a 58th-minute goal to restore Cambridge's lead and they extended it six minutes from time with an excellent effort from Devon White.
County rallied again but all they had to show for their frenzied efforts was an 87th-minute goal by Tony Agana.
Cambridge's caretaker-manager, Gary Johnson, said: 'This is the start of a new era and it has set off on the right note. The chairman has said it's my job until I prove otherwise and I'll stand or fall by what I do.'
Scarborough's John Ashdjian grabbed an 87th-minute equaliser to earn a replay with Plymouth on 11 November after their 3-3 draw at Home Park.Reuse content