Rosa Maria Da Cruz, a mother of four originally from Portugal, was given a two-year jail sentence and three years suspended by the court in south-west France.
She is also to receive psychiatric treatment and be monitored by social services for five years.
Her daughter, Serena, was discovered by a car mechanic when she took it to be repaired.
Hearing a noise, he opened the trunk to discover the baby in a car seat, naked, filthy and dehydrated.
She was surrounded by maggots and excrement.
The child was also kept in an unused room at the family home in the Corrèze region.
Da Cruz's lawyers said she had never accepted falling pregnant again.
Her partner, unemployed bricklayer Domingos Sampaio Alves, insisted he had no idea she had given birth to another child.
On Friday Da Cruz asked her daughter's forgiveness.
"I realise I hurt her a lot and that I will never again see my little girl," she said.
She and Alves were allowed to keep their three older children, whom a paediatrician described in court as having been "perfectly raised".