M. Untel & Mme. Untelle

from french.about.com Tel Telle Tels Telles

If we are looking for a placeholdername consisting on a [First Name + Name] basis the nearest equivalent of “John Doe” would be Jean Dupont, although the combination of Jean+Dupont is not as consistent in French as John+Doe is in English. Variations are found on first name (Pierre for example), or on name (Durant or Durand for example)

But the use of the equivalent of “John Doe” in French really depends on the context.

At the morgue and in legal matters in general a “John Doe” will be referred to as inconnu (inconnue in the case of an unidentified female).

In sociological studies and the press it is usual to designate a “John Doe” as l’homme de la rue.

In the press and in literature it is usual to come upon phrases such as
Monsieur Toutlemonde, Monsieur Dupont, Tartempion (colloquial), …

On forms that have to be filled in (administration, online forms…) we sometimes find Dupont or Jean Dupont.

For reference on the Royal Bank of Canada Website :
“Here lies the mind of John Doe” → “Ci-gît l’intelligence de Monsieur Dupont“.

When females then Madame Toutlemonde, Madame Dupont, Marie Dupont, la femme de la rue…

When seeking this information in the past I came upon a Wikipedia page with an interesting breakdown and was unfortunately the most substantial I could find at the time, especially since my knowledge of French is abysmal despite trying to write a piece featuring French characters in late 19th/ very early 20th century.

As others mentioned, it really depends on what you are using the placeholder for:
https://en.wikipedia.org/wiki/List_of_placeholder_names_by_language#French

I’m sure I don’t have to say this is merely a good jumping-off point and not definitive information. Hope this helps.