Young earned the honour in part for helping make Portman a red carpet fixture last year while she was pregnant and on her way to winning the best actress Oscar for "Black Swan."
The issue ranks the trendsetters behind the A-listers, including Leslie Fremar at No. 2, whose clientele includes Charlize Theron and Reese Witherspoon.
No. 3 is Petra Flannery, who likes to discover new talent and make them fashion stars. Her roster includes Emma Stone, Zoe Saldana, Mila Kunis and Megan Fox.
Other notables: At No. 11, Jeanne Yang, who styles Katie Holmes so well they have a luxury sportswear collection in their names at Barneys New York; No. 12, L'Wren Scott, who is engaged to Mick Jagger and a favourite of first lady Michelle Obama; and No. 20, reality TV star Brad Goreski, who made the list while his former employer, stylist-turned-designer Rachel Zoe, went from No. 1 last year to off the countdown this year. (THR says she's now "too big to rank.")
The list is available online, and the magazine hits newsstands Thursday.