The singer urged her fans to cast their vote after publicly coming out in support Jeremy Corbyn’s party.
It is a boost for Labour with the influential star pushing the youth vote head of polling day.
The 26-year-old, who was born in South Shields, South Tyneside, believes in funding the arts and fighting climate change.
She said coming from a working class background: “I know it’s important that we have a government which supports everyone and not the few”.
In a statement shared on Little Mix’s official Twitter account, Thirlwall added: “We need to save our NHS. Labour believes in funding the arts. They support the fight against climate change. Their manifesto supports equality and the LGBT community.
“Whoever and however you vote tomorrow, be proud of your choice and use your voice.”
Thirlwall shared a picture of herself alongside the statement, showing her wearing a jumper bearing the Labour slogan “for the many not the few”.
The singer, who is part of chart-topping pop group Little Mix alongside Jesy Nelson, Leigh-Anne Pinnock and Perrie Edwards, also urged fans to cast a vote on Thursday.
Writing to Little Mix’s 11.9 million Twitter followers, she said: “I grew up knowing the importance of taking part in our society and realising that we all have a choice and we all have a voice.
“I want to use my platform to encourage you to do the same and use your voice tomorrow in the General Election.
“Young people are less likely to vote but it’s so important that you do and have a say in the future of your country. Who you vote for is your decision but whatever you do, make sure you use your vote.”
Polls have suggested the gap between Boris Johnson’s Conservatives and Mr Corbyn’s Labour has tightened in the days leading up to the vote.