FA Cup wrap: Chelmsford, Coventry progress
A brace from Donovan Simmonds earned hosts Chelmsford a 2-0 lead at Melbourne Stadium, before Michael Rose pulled one back for League One visitors Colchester.
But the unheralded hosts were not to be denied, Jamie Slabber netting in the 89th minute to see the team from the Conference South - the fifth tier of English football - progress to the second round.
Of the fallen giants in action, 1987 FA Cup winners Coventry City defeated Arlesey Town 3-0 and Sheffield United came from behind to win 2-1 away to Bristol Rovers.
Preston North End, whose last FA Cup triumph came in 1938, saw off visiting Yeovil Town 3-0.
But 2008 FA Cup winners Portsmouth hit another low, as the financially stricken club lost 2-0 at home to Notts County.
Francois Zoko and Yoann Arquin struck for the away side in an all-League One clash at Fratton Park, where the hosts finished with 10 men following the dismissal of Scott Allan with 16 minutes to go.
The loss comes just four years after Portsmouth won the competition, a high point in their recent history before going bankrupt and sliding down the divisions.
League One side Crawley Town could only manage a narrow 2-1 win away to Metropolitan Police FC of the Isthmian Premier Division, effectively the seventh tier of English league football.
In other matches, Rotherham saw off Stevenage 3-2 at home, while Harrogate Town beat Torquay United 1-0 on the road.
Alfreton Town struck twice in injury time to beat Wrexham 4-2 away from home, despite finishing the tie with nine men after two of their three goal-scorers were shown red cards.
Both teams were awarded penalties in the match at the Racecourse Ground, while Wrexham also had a man sent off.
Other notable results saw Will Hatfield bag all four goals as Accrington Stanley beat AFC Fylde 4-1 away, while there were six different scorers as Chesterfield hammered Hartlepool 6-1 at home.
Conference side Macclesfield Town staged an upset, beating Paolo di Canio's League One Swindon Town 2-0 away from home.