Grégoire was born on April 24, 1975, in Montreal, Quebec, the only child of Jean Grégoire, a stockbroker, and Estelle Blais, a Franco-Ontarian nurse. Her family lived north of the city, in Sainte-Adèle, eventually relocating to Montreal when she was four years old. She was raised thereafter in Montreal's Mount Royal suburb, where she was a classmate and childhood friend of Michel Trudeau, the youngest son of Prime Minister Pierre Trudeau and brother of Grégoire's Future husband, Justin Trudeau.

Grégoire's first job was as a receptionist and assistant at an advertising firm. She was promoted to an account manager, but after three years of working in advertising, public relations, and sales, she decided to attend radio and television school, where she immediately knew, "I had found my calling". After completing her studies there, Grégoire got a job in a newsroom, writing the news ticker. A lover of culture, arts, and films, when she became aware of an opening at Quebec television station LCN for an entertainment reporter, she applied and was successful in obtaining the position. In addition to serving as an entertainment reporter for LCN's daily Showbiz segment, she contributed to segments on Salut Bonjour Weekend, Clin d'œil, and Bec et Museau for TVA, and hosted Canal Évasion's Escales de Rêves and Canal Z's Teksho. Grégoire also served as co-host on CKMF Radio's morning shows and contributed to Radio-Canada's Coup de Pouce. Additionally, she worked in the mid-2000s as a personal shopper for upscale department store Holt Renfrew.

Grégoire first met Justin Trudeau, the eldest son of Prime Minister Pierre Trudeau, when they were both children growing up in Montreal, where Grégoire was a classmate and childhood friend of the youngest Trudeau son, Michel. Grégoire and Trudeau reconnected as adults in June 2003, when they were assigned to co-host a charity ball, and began dating several months later. They became engaged in October 2004, and married on May 28, 2005, in a Catholic ceremony at Montreal's Sainte-Madeleine d'Outremont Church. They have three children: Xavier James Trudeau (born October 18, 2007), Ella-Grace Margaret Trudeau (born February 5, 2009), and Hadrien Grégoire Trudeau (born February 28, 2014).

As part of her work with WaterCan, which aims to alleviate global poverty by providing clean water to underdeveloped countries, Grégoire travelled to Ethiopia in October 2006 with her mother-in-law, Margaret Trudeau, who is the honorary President of the organization. Their trip was featured in a CTV documentary, "A Window Opens: Margaret and Sophie in Ethiopia", which aired in May 2007.

After her husband became a Member of Parliament for Montreal's Papineau riding in 2008, Grégoire continued to live in their Montreal home with their children, while Trudeau stayed at a hotel in Ottawa during the week. In June 2013, two months after Trudeau became the leader of the Liberal Party of Canada, the couple sold their home in Montreal's Côte-des-Neiges neighbourhood and began living in a rented home in the Rockcliffe Park area of Ottawa.

Grégoire is fluent in French, English, and Spanish. She became a certified yoga instructor in 2012.

Grégoire Trudeau's husband was officially sworn in as the Prime Minister of Canada on November 4, 2015. After the end of the 2015 election, Grégoire Trudeau indicated her preference for the hyphenated surname of "Grégoire-Trudeau", but she switched to using the unhyphenated form in March 2016.

On January 18, 2016, Grégoire Trudeau made the impromptu decision to sing a work she composed, entitled "Smile Back at Me" at the end of a speech in honour of Martin Luther King, Jr. Day at Ottawa City Hall. Mike Strobel, of the Toronto Sun, said the crowd gave her a standing ovation. Toronto Star pop music critic Ben Rayner said the song was "out of pitch" and "cast serious doubt upon her musical judgment". In Maclean's, Michael Barclay described the song as "fine", although not designed to be sung a capella, and that Grégoire's voice was "surprisingly strong". Remixed versions of the song were released online afterward.