It also won best comedy actress for its star, Saoirse Ronan.
Gerwig's directorial debut won the Golden Globe Award for best film comedy or musical.
Nominated in six categories, Martin McDonagh's Three Billboards Outside Ebbing, Missouri was the biggest film victor on the night, winning four awards, including Best Motion Picture - Drama.
Saoirse Ronan's mother watched her pick up a Golden Globe award via FaceTime. Many in the industry believe that Gerwig missed a well-deserved nomination in the director category at the Globes, which was won by Guillermo del Toro for "The Shape of Water". Her breakthrough came with the part of a precocious teenager in Atonement (2007), for which she was nominated for the Academy Award for Best Supporting Actress.
"I have no time at all to say thank you but I just want to say how inspirational it has been to be in this room tonight".
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Saoirse - who wore black in support of the Time's Up anti-harassment initiative - went on to thank "all of the women" in her life who she loves and who support her "every single day", again mentioning her mother, as well as her best friend Eileen, who was her date for the evening.
"So happy birthday, mum - even though I think she wanted Lady Bird to win!"
Speaking to British Vogue, Saoirse's stylist Elizabeth Saltzman revealed that she and the actress decided on this early on.
The picture bested Jordan Peele's "Get Out", James Franco's "The Disaster Artist", Michael Gracey's "The Greatest Showman", and Craig Gillespie's "I, Tonya". The actress is up for Best Performance by an Actress in a Motion Picture - Musical or Comedy.
The Golden Globes are on RTÉ2 on Monday night from 9pm.