The 14-year-old Londoner overcame the 16-year-old Thai third seed in one hour, 34 minutes after an impressive performance on Court One.
Robson follows in the footsteps of 1984 champion Annabel Croft, the last Briton to win the girls' title.
"It was so good today as all the crowd were behind me and it was an overwhelming experience," said Australian-born Robson.
"I didn't feel relaxed and I thought I was going to be sick when I walked on court as there were so many people watching.
"In the second set I went a bit mad but I got it back together and that is how I won."
Left-handed Robson claimed an early advantage when she broke in the second game of the match. She served out for the opener in just 29 minutes to the pleasure of a packed home crowd.
The two players traded breaks at the start of the second set and again in the seventh and eighth games.
From there the Thai took advantage of a string of double faults and unforced errors from her rival to break again in the eighth before serving out for the set.
After three more breaks of serve to start the third set, Robson saved a break point in the fifth game and held her nerve to go ahead 4-1.
She broke again in the sixth game as Lertcheewakarn sent an unforced backhand error wide.
Robson hit another double fault on triple championship point, before wrapping up the victory with a service winner.
Recent Wimbledon junior champions include Martina Hingins, Amelie Mauresmo, Katarina Srebotnik, Agnieszka Radwanska and Caroline Wozniacki.