The Bab wrote that He is the Qaim, and the Mahdi. When the Bab was martyred, Bahaullah said , He is the reappearance of the Bab, meaning after the Bab, Bahaullah was the Mahdi.
In Hadithes it is written that "No Mahdi except Christ". In my view, this is certainly an authentic Hadith. Thus, Bahaullah was both the Mahdi and Christ.
Also @Trailblazer
It was offered to @paarsurrey that to understand how Baha'u'llah is the Mahdi, does requires the reader to understand what Baha'u'llah offerd.
Baha'u'llah offered that all names and titles revert back to the Manifestation and Baha'u'llah is currently the focal point of all past Names, the whole purpose of their Message.
In the name Baha'u'llah, 'The Glory Of God', all Names are seen.
There are many works of Baha'u'llah still untranslated, but I just came upon this provisional translation.
I see it contains the information we are looking for;
http://iranpresswatch.org/post/740/bahaullahs-visitation-tablet-for-imam-husayn/
Regards Tony