The Kildare light welter beat the Californian on a split decision in the 64kg quarter final to book her ticket into the last four.
Phelan was straight into a medal fight following a bye, and hit the ground running against here Californian opponent.
The younger sister of featherweight pro Allan will now face Russian Ekaterina Dynnik in the semi finals on Friday.
The Russian is a European Youth bronze and silver medalist, and defeated Ireland’s Shauna O’Callaghan on a split for European Junior gold in 2015
Team manager Anna Moore hailed Phelan’s performance noting that “she was absolutely outstanding and dominated from start to finish. We’re thrilled with the win.”
Belfast light fly Caitlin Fryers and Bray welter Shannon Reilly will be aiming to add to Ireland’s medal haul in the afternoon session.
AIBA Youth Women’s World Boxing Championships Guwahati, India
51kg Shauna Blaney (Ireland) lost to Ekaterina Molchanova (Russia) 0-5
48kg Caitlin Fryers (Ireland) beat Nasu Misaki (Japan) 5-0
69kg Shannon Reilly (Ireland) beat Olivia Hussey (England) 3-2
75kg Lauren Kelly (Ireland) lost to Ya Chun-Yang (Chinese Taipei) 0-5
48kg Caitlin Fryers (Ireland) beat Sumaiya Kosimova (Tajikistan) 5-0
54kg Mary Geraghty (Ireland) lost to Marie Bachmaier (Germany) 1-4
57kg Labhaoise Clarke (Ireland) lost to Namuun Monkhor (Mongolia) RETI3
60kg Marguerita Nevin (Ireland) lost to Klaudia Budasz (Poland) 2-3
48kg Caitlin Fryers (Ireland) v Karina Tuvakova (Russia)
64kg Katelynn Phelan (Ireland) beat Aidyl Cardenas (USA) split
69kg Shannon Reilly (Ireland) v Citalli Ortiz (USA)
64kg Katelynn Phelan (Ireland) v Ekaterina Dynnik (Russia)
48kg Caitlin Fryers (Immaculata)
51kg Shauna Blaney (Navan)
54kg Mary Geraghty (Baldoyle)
57kg Labhaoise Clarke (Carndonagh)
60kg Marguerita Nevin (St.Brigid’s, Kildare)
64kg Katelynn Phelan (St.Brigid’s, Kildare) (Bronze, at least)
69kg Shannon Reilly (Bray)
75kg Lauren Kelly (St.Brigid’s, Edenderry)
Team manager: Anna Moore
Coaches: Jimmy Payne, Liam Brereton, Antia Just
R&J: Sadie Duffy