Mitchell was consistent all the way through the competition in Solihull but sealed the title on the final combined run-shoot event.
She overtook Wycliffe’s Zoe Davison on the last event to finish with a total of 1,274 points.
Davison claimed silver with 1,259 points and Lancaster’s Emma Whitaker was third with 1,241.
Mitchell, competing against older girls, clocked 2:15.52 for the swim, claimed 22 victories in the fencing, recorded a time of 1:12.04 for the ride and 13.51.45 for the combined event.
Mitchell will be in action for Great Britain at the European Under-17 Championships in Bulgaria this weekend.
Joining her in the team will be fellow Plymouth student Annabel Denton.
The competition in Pazardzhik starts on Friday and continues through to the Monday.
Meanwhile, former Plymouth College pupil Myles Pillage missed out on reaching the individual final at the European Junior Championships. However, he will have a chance for success in the relay events which start on Thursday.