Mercey, 24, joined the Saints from Saracens in the summer of 2010. In his 18 months at Franklin's Gardens the prop, who has won honours with England Saxons, has seen plenty of game time in all competitions, with the 2011 Heineken Cup final just one of his 38 appearances.
"Moving to the Saints was good for me and I'm enjoying being at the club," said Mercey. "Playing in the Heineken Cup final is an obvious highlight, but running out and playing in front of the great Northampton fans has also been enjoyable over the past 18 months. There are some good players at the club and I'm concentrating on improving my own game."
Mogensen has yet to make his first team debut, but the 20-year-old winger has been identified as one to watch in the current Saints Senior Academy. Although his game time was limited in his first season as a full-timer at Franklin's Gardens, he has played plenty of rugby so far in 2011/12, predominantly on loan with Cambridge, for whom Mogensen scored nine tries in his 16 appearances. He is currently dual registered with Moseley and is stepping up his experience playing in The Championship.
"I had a frustrating first year, but everyone at the Saints was very patient and I'm enjoying my rugby this year," Mogensen said. "I've been learning a lot playing for Cambridge and Moseley and for the Wanderers, and training with the Saints during the week. Hopefully if I put in the work I can put myself into a position where I can contribute to the Saints first team in the future."
McMillan, another 24-year-old front rower, initially arrived at Franklin's Gardens this season on trial, the hooker having previously played for Birmingham and Solihull in The Championship. He then signed a short-term deal until the end of the current season, which has now been extended.
"I'd like to thank the coaches for giving me the opportunity at the Saints," said McMillan. "Being at the club has helped invigorate my rugby and it is enjoyable being part of a squad which is aiming high rather than battling relegation, which is what I've experienced over the past couple of years. It has also been good to be able to concentrate on my game full-time and I'm happy to be staying at Franklin's Gardens."
Director of rugby Jim Mallinder said that all three players were promising and provided added depth to the Saints squad.
"We need players who can contribute to the squad and this is why we've extended the contracts for all three players," he said. "Tom has been a regular part of the first team squads throughout his time at the club and we know that he can deliver good performances on a regular basis.
"Anders and Ross may not have had their first team opportunities, but they have both built up their experience in The Championship and are very eager to show what they can do in the black, green and gold."