Mookhey, 32, was elected by the Labor to replace Steve Whan in the New South Wales upper house, making him the state's first politician of Indian background, an ABC report said on Tuesday.
"It is an incredible honour and I am humbled to be the first Australian politician to take his oath of allegiance on the Gita," he said.
"My story is only possible because Australia is so open and so welcoming to the contributions of people like my parents and I'm thinking a lot about them today as I take my oath," Mookhey said, adding that the Gita is one of the world's great religious texts, along with the Bible, the Quran and the Torah.
"It was an important backdrop to my childhood growing up in western Sydney. I am really proud to sort of be using it as a mark of respect to the people that have helped me get to where I've gotten today, and I'm really very pleased and humbled by the opportunity," he said.
Mookhey said he hopes his time in the NSW Upper House is spent productively.
"I will certainly be a very loud voice for a much more modern state and much more modern city in which everyone is able to move around and spend time at work and with their family and not consumed with the sort of frustrations you have in urban cities such as Sydney," he said.
"That's really the issue closest to my heart and how I get to use this position that I've been so lucky and fortunate to get," he added.
Mookhey's parents had migrated from Punjab to Australia in 1973. Born in Blacktown suburb, Mookhey holds three university degrees and currently works as a consultant to unions, charities and community groups.