Mr Fillon, who turned 63 on Saturday, has seen his support dwindle after he disclosed he will face charges over allegations he gave his family fake parliamentary jobs.
In her first interview since the allegations emerged, Wales-born Penelope Fillon told Le Journal du Dimanche she had carried out a “lot of different tasks” for her husband and urged him to “keep going to the end”.
The Republicans candidate was the frontrunner until mid-January when the Canard Enchaine newspaper claimed he had paid his wife and two of their children nearly €900,000 (£780,000) as parliamentary assistants or advisors.
Under French law, politicians are allowed to employ family members but investigators are trying to discover evidence of what work she did carry out.
Mrs Fillon told the newspaper: “He needed someone to do a lot of different tasks, and if it wasn’t for me, he would have paid someone to do it, so we decided it would be me.
“I said to him that he had to keep going to the end. Every day I told him that.”
Mr Fillon has claimed the accusations are politically motivated.
But his own party has brought forward a meeting of its decision-making body to Monday to “evaluate the situation”.
It emerged on Friday that police had raided the Fillons’ chateau home near the northwestern town of Le Mans. The couple’s Paris apartment was also searched on Thursday.
Mr Fillon will attempt to reignite his campaign with a major rally on Sunday near the Eiffel Tower.
Opinion polls currently show that far-right leader Marine Le Pen and centrist Emmanuel Macron would go through to the second round of the presidential contest on 7 May, with Mr Fillon eliminated in the first round.
On Wednesday, Mr Fillon revealed he would meeting investigators on 15 March, when he will be placed under a formal investigation – the equivalent in France of being charged.
He had previously said he would step down if that happened, but has accused the judicial system of bias and vowed to continue fighting.
Supporters, including a campaign spokesman and five MEPs from the Republicans party, withdrew their backing this week and called for another candidate to be installed quickly.