"I have made a choice to do the best thing for our community's priorities, and importantly, for our country," she said.
Carney said in a statement of his own that Gladu is known to be willing to work across party lines.
"Having spent decades of her career in engineering and international business, she understands what it takes to create good jobs, strengthen supply chains, and ensure Canadian industry can compete in a rapidly changing global economy," Carney wrote.
Gladu has been elected as a Conservative four times in the Sarnia region since 2015.
She ran for the Tory leadership in 2020, after Andrew Scheer stepped down, but did not meet the qualifications to be on the ballot. The party chose Erin O'Toole as its leader that year.
Conservative Leader Pierre Poilievre named Gladu as the party's critic for civil liberties last May. She is also chair of the House committee on the status of women.
After the Liberals chose Carney as their new leader last March, Gladu posted a video on social media sharing her thoughts. She was highly critical of Carney, saying he was part of the "disastrous mess" of inflation that she blamed on Justin Trudeau's government.
"I don't see that he is going to bring any change at all," she said on March 15, 2025.
Gladu said in her statement Wednesday that she's "ready to work with Prime Minister Carney to build a stronger, more independent, and more competitive Canadian economy."
She also said her district of Sarnia—Lambton—Bkejwanong "was always a bellwether riding." It was last held by the Liberals from 1993 to 2006 and has been held by the Conservatives since then.
Gladu is the fifth opposition MP to join the Liberal caucus since November.
Matt Jeneroux, Chris d'Entremont and Michael Ma left the Conservative benches, while Lori Idlout crossed over from the NDP.
Gladu told a local media outlet in January that she believed MPs who leave their party to join another should quit their post and trigger a by-election. She supported a petition put forward by Conservative MP Lianne Rood that called for MPs to amend the Parliament of Canada Act to require floor-crossers to resign.
The Conservatives have decried the recent floor-crossings but have not put forward or formally supported any changes to laws that would prevent them from happening.
Carney's government now has 171 seats in the House of Commons, with three by-elections to come on Monday that could put them into majority territory.
The Liberals are widely expected to win in Scarborough Southwest and University—Rosedale, which were held by former cabinet ministers Bill Blair and Chrystia Freeland, respectively.
The race in the suburban riding of Terrebonne, north of Montreal, is a tight one. The Liberals are fighting to win a seat that traditionally has been held by the Bloc Québécois.
Carney told reporters Wednesday that the country "is uniting" and Gladu is "going to be a great member of our team, lots of energy, ideas, advice on a wide range of issues."
When a reporter pointed out that his government is closer to a majority, he replied, "What's important is that we command the confidence of the House."
Kim Fyfe, head of the Liberal riding association for Sarnia—Lambton—Bkejwanong, said the association did not know about Gladu's plans in advance but hopes to meet with her in person at the Liberal convention this weekend.
"We really haven't had a chance to process or discuss it yet. I only found out because people are sending me texts, emails in the last 45 minutes," Fyfe said.
"We'll be working with Marilyn. She's been our opponent in the last number of elections but now we’ll be working with her to encourage Liberal values in our riding and try to co-operate and work together on this."
A representative for the local Conservative riding association, reached by phone, did not provide a comment and directed media inquiries to an email address for the riding association.
Poilievre's office did not immediately respond to a request for comment.
Gladu's departure comes a day after Poilievre's director of communications, Katy Merrifield, told caucus that she will be leaving her post after Friday.
This report by The Canadian Press was first published April 8, 2026.
