Accepting the award, Zellweger thanked "my immigrant folks who came here with nothing but each other and a belief in the American dream."
"Our heroes unite us," she said during her 4-minute speech, in which she paid tribute to the Wizard of Oz star. "When we celebrate our heroes, we're reminded who we are."
To win, Zellweger saw off competition from Scarlett Johansson (Marriage Story), Cynthia Erivo (Harriet), Saoirse Ronan (Little Women) and Charlize Theron (Bombshell).
The award represents Zellweger’s second Oscar victory, having picked up a Best Supporting Actress trophy in 2004 for Cold Mountain.
She also received Best Actress nominations in 2002 and 2003, for her roles in Bridget Jones’ Diary and Chicago.
Zellweger's turn as Judy Garland has already landed the actress some prestigious awards, including the BAFTA for Best Actress.
For the second year running, the Oscars had no presenter. Nominations-wise, Joker led this year’s pack with 11, while The Irishman and Once Upon a Time in Hollywood followed close behind with 10.
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