MGM child star Margaret O’Brien, Tinseltown crier of the 1940s, won lasting fame as Judy Garland’s little sister Tootie Smith in the movie musical Meet Me in St. Louis.

The film’s most iconic moment: O’Brien, then age 7, sobbing as Garland introduces the instant holiday classic, Have Yourself a Merry Little Christmas.

O’Brien, now 74, cheerily recalls she had no trouble shedding real tears:

“ June Allyson and I had a competition, who could cry best on the MGM lot,” O’Brien says. “ That made me cry. ‘June’s not going to get ahead, I’m going to win the competition!’ ”

Old-time Hollywood directors often told terrible tales to child actors to get them to cry on-screen, but not to O’Brien.

“Judy and my mother would never have allowed anyone to tell me my dog died or anything like that,” she says. “ Jackie Cooper, he never quite got over that, he told me at one time.”

Garland, 21 when she made Meet Me in St. Louis in early 1944, was extremely protective of O’Brien, once telling dancers on the set, “That little girl isn’t having any childhood, and I know what I’m talking about,” according to John Fricke, author of the new book, Judy: A Legendary Career in Film.

O’Brien denies having an unhappy childhood, saying her mom, Gladys — unlike Garland’s notorious stage mother, Ethel Gumm — was not a real-life Wicked Witch of the West.

“My mother was wonderful,” O’Brien says. “My mother said, ‘My goodness, my child has to be protected.’ Judy’s mother wasn’t as good at that. . . .I was very lucky.”

Meet Me in St. Louis, directed by Garland’s soon-to-be second husband, Vincente Minnelli, will be released Dec. 13 on Blu-ray in high definition, with a remastered five-channel soundtrack (Warner, $36). The film is introduced by Garland and Minnelli’s only child, Liza.

Fricke describes the film as one of Garland’s “big four.” The others: The Wizard of Oz, Easter Parade and A Star is Born.

In Meet Me in St. Louis, Garland sang three of her best-known songs, all written by Ralph Blane and Hugh Martin: The Boy Next Door, The Trolley Song and Merry Little Christmas, a ballad reflecting the Smith family’s sad impending move to New York.

O’Brien says Garland didn’t like Martin’s original Christmas lyrics.

“Very dark,” O’Brien recalls. “Judy said, ‘I don’t want to sing something so dark, with little Margaret so upset about leaving for New York.’ ”

The original lyrics: “Have yourself a merry little Christmas; It may be your last; Next year we may all be living in the past.”

According to O’Brien, Garland helped Martin rewrite the words: "Have yourself a merry little Christmas. Let your heart be light. Next year all our troubles will be out of sight.”

“She would never ask for credit,” O’Brien says. “Thanks to Judy and Hugh Martin, we have a wonderful Christmas song every year.”

Steve Rothaus