The vegan advocate scooped the Academy Award for Best Actor for his role in acclaimed movie Joker.
After the awards, he and Mara headed to vegan eatery Monty's Good Burger - which has three LA outlets - and enjoyed a meal.
Monty's shared a snap of the happy couple dining, with the caption: "Hey Joaquin, you just won an Oscar for best Actor, what are you going to do next?"
Phoenix made major headlines with his Oscar speech, in which he condemned the dairy industry, saying his platform gives him the 'opportunity to use our voice for the voiceless'.
"I think at times we feel or are made to feel that we champion different causes. But for me, I see commonality. I think, whether we’re talking about gender inequality or racism or queer rights or indigenous rights or animal rights, we’re talking about the fight against injustice," Phoenix said.
"We’re talking about the fight against the belief that one nation, one people, one race, one gender, one species, has the right to dominate, use and control another with impunity.
"I think we’ve become very disconnected from the natural world. Many of us are guilty of an egocentric world view, and we believe that we’re the center of the universe. We go into the natural world and we plunder it for its resources. We feel entitled to artificially inseminate a cow and steal her baby, even though her cries of anguish are unmistakable. Then we take her milk that’s intended for her calf and we put it in our coffee and our cereal.
"We fear the idea of personal change, because we think we need to sacrifice something; to give something up. But human beings at our best are so creative and inventive, and we can create, develop and implement systems of change that are beneficial to all sentient beings and the environment."