Now a svelte, beautiful actress of 37, Gilbert won the election over three other candidates in a surprising upset over actress Valerie Harper, the favorite.
But Harper, and the other candidates, Angel Tomkins and Eugene Boggs, did not have the advantage of Laura Ingalls' aura going for them.
Laura, you remember, was the delightful moppet of "Little House on the Prairie," the buck-toothed, befreckled darling who became the centerpiece of the enormously popular, long-running TV drama.
Americans, including almost all SAG members, fell in love with the adorable tot with wide-set eyes, stubborn chin and magical charm that bewitched us all.
She wore sunbonnets, aprons and brogans as a 19th century farm child who could provoke tears as well as peals of laughter in her performances. But most of all she engendered love from viewers of every age.
All this was possible because Melissa Gilbert, an extraordinary actress, infused Laura with a living presence of innocence, sweetness and pluck.
For a dozen primetime years (1983-74) Gilbert and the show's star, Michael Landon, convinced the world they were father and daughter in the bewildering world of the American frontier.
So popular was Melissa, she became the youngest person ever to receive a star on the Hollywood Walk of Fame.
Just as she mesmerized generations of viewers, Melissa continued to attract audiences in mature TV movies and feature films.
From an adorable, amusing little duckling, Gilbert developed into a slender, graceful swan, appearing in adult roles, all of which required sophisticated acting.
This week a radiant Gilbert was somewhat breathless over her election to the SAG presidency.
At her elegant home overlooking the family stables and five acres of rolling California hills, Gilbert was still aflutter about her surprising victory.
"I'm still thrilled over becoming Madame President," she said, laughing. "I was stunned."
What could have inspired an exceedingly busy wife, mother and actress to enter the political arena of one of the country's largest unions (100,000)?
"It was a calling," she said. "I had no choice. It was my time.
"My youngest son is now in kindergarten and I wasn't working at the moment. But I still can accept roles while I'm president; and here I am.
"I'm the president of a union! I'd been on the board of directors for a year and thought it was time for new leadership and I stepped in.
"I won by more than a thousand votes.
"Valerie (Harper) was endorsed by (then) president Bill Daniels. Everything I said during the campaign was the opposite of what he said.
"There was a silent majority of SAG members who were very dissatisfied with the hard line of the past leadership. They were ready for something a little more moderate.
"But I want to focus on the future, and I would hope that wouldn't involve any strikes."
Will it be difficult for Melissa to discharge her duties as SAG president with her acting career?
"There are 11 vice-presidents in place. And this isn't a paying job, so we're allowed to accept acting work. They make that concession.
"I'm gone five weeks here, six weeks there when I'm doing a TV film, so it isn't that long."
Melissa is elated to be the third female president of SAG, preceded by her friend Patty Duke and Kathleen Nolan.
The first call I received the morning after election was from Patty who said, 'Of course you won!'
"Patty had been very supportive and encouraged me to run. I have yet to hear from Valerie or any of the people who didn't support me.
"I've got so much to deal with, including the ATA problem. The Association of Talent Artists franchise agreement is up and we're in the middle of negotiations.
"I need to get into that. The contract expires at the beginning of January. There are other pressing issues ahead of me.
"I told my husband (actor Bruce Boxleitner) he'd better be nice to me or I'd mess with his pension. He's very happy and supportive. So are our sons, Dakota, Michael, Sam and Lee."
Does Melissa think her victory may be due partly to her personal popularity and everyone's affection for little Laura Ingalls?
"I would like to hope not but I think if you look at all the people who did get elected, they definitely went with the best-known celebrities rather than someone who was less well-known.
"Obviously they didn't think I was too young and too sweet. I don't know.
"Winning has already changed my life. It's crazy around here today. The phone never stops ringing.
"I spent the last two weeks hanging out with my family and getting energized and now I'm ready to go."