The right-back most recently played for Fleetwood Town, making over 200 appearances in a five-year spell with the Cod Army.
McLaughlin has also won 22 caps for his country, scoring his first goal for the side in a 4-0 win over Azerbaijan in November 2016.
"I'm delighted to get everything signed," the 25-year-old told the official Millwall website.
"I first heard of Millwall's interest towards the back end of last season, but I then went away on holiday and got married, so it took a bit of time - but I'm glad it's all sorted now.
"I'm really looking forward to getting going. I just want to start and get to know my teammates."
Lions boss Neil Harris added: "We've been chasing Conor all summer. He was one of the outstanding performers in League One last season and was voted by his peers into the Team of the Year - which is the biggest accolade you can have in professional football.
"He's a solid, reliable right-back, who is first and foremost a very good defender. He's technically very good and has many caps for Northern Ireland. His character will blend in very well with the rest of the group."