A first Lieutenant of SS in WWII, Montana was ordered by Hitler to lead the project of creating an army of Nazi vampires. Upon the defeat brought by Alucard and Walter in Poland, Montana and his officers escaped to South America. The original purpose of the Vampire project was to ensure Germany victory in WWII, but Montana now has his own agenda, preaching of endless war and destruction, and seems to pay no heed to who will be victorious.
Montana claims to be human, although he has somehow managed to retain his youth since WWII, possibly by alterations done by Doc.
It appears from Hellsing Ultimate OVA 10 that Montana's Body for the series was that of a robot and he may have in fact been long dead.