I'm sorry but I really don't remember much. I came across this series once searching randomly for a good shoujo manga.

It is a shoujo about a girl who comes across a guy who is in mafia or one of the nominated to be the next head. If it rings any bells, I'll be really very grateful

Without more information anything I can guess would be a wild stab.
Check out http://www.mangaupdates.com/series.html?category=Mafia& genre=Shoujo&perpage=50 that's the best help I can give with so little to go by.
(And if it's not there, either it's not tagged with the mafia tag, or maybe it was something similar like the yakuza, just a gang.)

Can you remember anything else? The length of the girl's hair (or guy's) . If anyone wore glasses. Did everyone know about him, and his connections to the mafia. How they met?
Was it a oneshot, or at least a volume long. Still ongoing? (Last you knew)
Every little bit helps, even if it doesn't seem like useful information.

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Thanks a lot but I just found it.
Inchou no Himegoto

I was searching the database with the mafia keyword and it helped me to narrow down my options.
Thanks a lot for your help