That role came to a messy end on Saturday when her partner President Francois Hollande announced they had split two weeks after a scandal erupted over his affair with actress Julie Gayet.
The feisty 48-year-old was herself once the other woman, remaining Hollande's secret companion as he kept up appearances with ex-partner Segolene Royal for her failed 2007 presidential campaign.
That same year Hollande split with Royal, the mother of his four children, and his relationship with Trierweiler was made public. In October 2010 Hollande, then the head of the Socialist Party, told the popular magazine Gala: "Valerie is the love of my life."
The twice-divorced Paris Match journalist emerged into the spotlight during Hollande's presidential campaign and warned she would not be a wallflower, saying in April 2012: "I have character, they cannot muzzle me."
These words quickly rang true, as the elegant brunette ran into trouble just a few weeks after Hollande's victory when she tweeted her support in legislative elections for the independent candidate standing against Royal.
To the French, the tweet was a zinger aimed directly at her former love rival, who had shared Hollande's life for three decades, and Trierweiler's reputation suffered, with many deeming her haughty and arrogant.
Four months later Trierweiler apologised, saying she had made a mistake "that won't happen again".
Although France does not have an official first lady, Trierweiler -- dubbed the First Girlfriend by US media and Rottweiler by her detractors -- did have a small staff at the Elysee presidential palace, including a chauffeur.
After Hollande's election, Trierweiler cut down her work at Paris Match, occupying herself with charitable work.
She had also previously worked for the private television station Direct 8, doing interviews and presenting a political show.
Hospitalised after news of affair
But her life in the Elysee Palace began to unravel when on January 10, the glossy magazine Closer published pictures of Hollande visiting his 41-year-old lover. Later reports said they had been seeing each other since before his election.
The news hit Trierweiler hard, and she checked into hospital for what was reported as "fatigue" on the same day the report came out, remaining there for a week.
The first question posed to Hollande at a Paris press conference a few days later was "is Valerie Trierweiler still the first lady?"
Hollande stalled on clarifying the situation for two weeks, announcing the split the night before Trierweiler was due to emerge from her rest at a presidential retreat outside Paris for a charity trip to India.
Trierweiler's only response to the scandal came after her release from hospital on January 19, when she tweeted her thanks to her supporters from her personal account. "Very touched," she wrote.
While France demanded clarification on her official status, Trierweiler won sympathy from former first lady Bernadette Chirac, whose husband Jacques had confessed to a roving eye.
Chirac said she "shares her sorrow," and added that being in the limelight and first lady was not easy.
Born on February 16, 1965 to what she termed "a bourgeois family that had lost its fortune" and lived in a modest neighbourhood, Valerie Massonneau Trierweiler is the fifth of six children.
While her grandfather had run a bank at one time, her father was physically handicapped and after his death, her mother worked at a local ice-skating rink to make ends meet.
Originally from Angers, western France, she made her way to Paris to study political science and communications at the Sorbonne university, where she met and married Denis Trierweiler, with whom she had three sons before their divorce.