He has represented Ireland at Schools, Under-19, Under-21, Sevens and 'A' levels but his progress has been halted by injuries, including a bad knee injury in 2004 which kept him out for a year.
Commenting on his move to 'Quins, he said: "I have been with Ulster for seven years and, although I have had a great time at the club, I feel that it is the right time to take the next step. 'Quins will be a new and exciting challenge.
"I respect everything that Dean (Richards) and his squad have achieved so far this season. Harlequins is a young and dynamic team and a I am looking forward to being a part of it next season."
Harlequins coach Dean Richards added: "I have known Neil since his early days at Leicester, and I am looking forward to being a part of his future development.
"Neil is a genuine number 7. He is a great tackler with a huge work rate."
McMillan will begin training with the 'Quins squad at the end of the season.