The partnership ended when Watling was out to the last ball before tea, caught by Rory Burns at gully from the bowling of Stuart Broad for 55.
Batting first after losing the toss, New Zealand was 315-6 at the break with Mitchell 63 not out.
Mitchell, on test debut, came to the crease after Watling but reached his half century first, in 194 minutes from 122 balls with five fours and a six. The six was probably the most dramatic shot of the day, a straight hit from the bowling of part-time leg-spinner Joe Denly which carried 110 meters on the fly.
Mitchell is one of six members of the New Zealand 11 for this match — Watling, Jeet Raval, Kane Williamson, Henry Nicholls and Tim Southee are the others — to have scored a half century or better on debut.
He comes from rugby royalty. His father John Mitchell was an All Black, an All Blacks head coach and is the current defensive coach of the England rugby team.
The 28-year-old allrounder played nine Twenty20 internationals before earning a test debut on his home ground as a replacement for the injured Colin de Grandhomme.
He joined Watling Saturday morning after England captured the wickets of first day century-maker Tom Latham for 105 and his overnight partner, Nicholls (9), within nine overs of New Zealand's resumption at 173-3.
Watling and Mitchell batted through most of the first session — extended because of the rain which washed out the final session Friday — and all but the last ball of the second session in an imposing partnership.
Watling's half century continued an extraordinary run of form in tests. He has scored 77, 105, 205 and 55 in his last four innings, batting more than 21 hours in the process.
Mitchell will be joined after the break by Mitchell Santner, who made a maiden test century in New Zealand's first test win by an innings and 65 runs.