Longoria plays cheating housewife Gabrielle Solis in Wisteria Lane and at 30 is the youngest member of the mostly 40-something cast of the latest US hit, which also stars Marcia Cross, Teri Hatcher and Felicity Huffman.
According to a report in The Sun, Longoria was told "she was picked for her stunning face and figure but also because of her glossy hair. Eva is absolutely thrilled and looking forward to getting started."
She will join Catherine Deneuve, Claudia Schiffer, Andie MacDowell, Jennifer Aniston and Natalie Imbruglia, who have also appeared in advertising campaigns for the cosmetics giant.
The star recently complained in an interview that she got little attention from photographers when the 'Desperate Housewive' cast are shot because she is too good looking.
"I always have this problem at photo shoots, when all of us are together. I'll get the least attention. They say, 'She'll look good in anything.' Or, 'Oh, she doesn't need the best lighting, because she'll look good in any light.'"
She went on in the US Marie Claire interview to say that as the "sexy" member of the cast she found it hard to stay in shape.
"I find it's harder to stay in shape, especially with my character being the sexy one and all that," Longoria said. "I just want to try and establish habits now that will carry me through my 30s."
The £1m deal is dwarfed by the £2.6m deal L'Oreal paid singer Beyonce Knowles last year for just 10 days work a year in a five-year deal to promote its haircare products.
McCann Erickson handles L'Oreal's advertising internationally. The brand is famous for its "Because you're worth it" strapline and its many variations.
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