Porter, 31, arrives after being released by the Blades, having spent three and a half years with the club, scoring 23 times in 118 games.
Briggs, 23, has made 12 appearances for Millwall this season.
"It's an opportunity I jumped at and I've been pleasantly surprised by the facilities," he told BBC Essex.
"I'm here until the end of the season and I believe we have the quality to get out of trouble."
Briggs started his career at Fulham, where he became the Premier League's youngest ever player by making his debut aged 16 years and 65 days.
Porter, who scored four times as the Blades reached the FA Cup semi-final last term, has featured for Bury, Oldham, Motherwell and Derby, but has not played a game since November.
"It's been frustrating not playing at Sheffield United because I signed a new contract in the summer," he said.
"This is the first time I've moved south, so it's a big move for me."
Both players are expected to feature against Leyton Orient on Saturday.
Colchester have signed Sheffield United striker Chris Porter on an 18-month deal and Millwall defender Matthew Briggs on loan for rest of the season.