Mitchell has not played since suffering a serious knee injury while playing for an England XV against the Barbarians in June.
But the scrum-half, who won Saints' young player of the year award last season, has been training with England's Six Nations squad in recent weeks.
And he is now fit enough to be named on the bench at Franklin's Gardens this weekend, with Cobus Reinach retaining the No.9 shirt.
Matt Proctor returns to action following the concussion that has kept him out since early January.
The Kiwi centre teams up with Rory Hutchinson, who is back after being on the bench for Scotland against England last Saturday.
Harry Mallinder will make his first Premiership appearance for 15 months as he lines up at full-back.
Tom Collins returns to the starting line-up for the first time since his suspension in January as he replaces Ollie Sleightholme on the wing.
James Grayson is at fly-half, with Dan Biggar having suffered concussion while on Wales duty last weekend.
In the pack, Teimana Harrison starts at No.8, making his 150th Saints appearance in the process.
Mitch Eadie, who will join Bristol this summer, starts in the six shirt.
Alex Moon is back from England duty and lines up alongside fellow Academy graduate Alex Coles in the second row.
Lewis Bean provides cover from the bench, with Devante Onojaife and Fraser Dingwall, who has recently been with England, also among the replacements.
But Saints are without Jamie Gibson (ankle), David RIbbans (ankle), Piers Francis (concussion, Sleightholme (hamstring), JJ Tonks (neck) and Api Ratuniyarawa (suspended).
Bristol have been handed a big boost with key man Charles Piutau able to make his return from injury.
The full-back is the only change to the side that beat Gloucester last time out as Bristol target back to back wins in the Gallagher Premiership.
Luke Morahan switches to the wing, while Shaun Malton continues at hooker with Harry Thacker (neck) unavailable.
Saints: Mallinder; Collins, Proctor, Hutchinson, Naiyaravoro; Grayson, Reinach; A Waller (cc), Haywood, O Franks; Coles, Moon; Eadie, Wood, Harrison (cc).
Replacements: Fish, B Franks, Hill, Bean, Onojaife, Mitchell, Dingwall, Tuala.
Bristol Bears: C Piutau; Morahan, O'Conor, Bedlow, Leiua; Sheedy, Randall; Woolmore, Malton, Afoa; Attwood, Vui; Luatua (c), D Thomas, Hughes.
Replacements: Capon, W Thomas, Thiede, Joyce, Heenan, Uren, Madigan, Purdy.