What is the basis that many mainstream Christian groups justify labeling groups like the Mormons and Jehovah Witnesses as Non Christians? If there is no clear definition stemming from the Bible of what makes one a Christian, how does mainstream Christianity justify labeling groups like the Mormons and Jehovah Witnesses as Non Christians?

Members from both groups seem to take a more active role in spreading Christ's message of salvation than do the most rooted mainstream Christian church members, based off of their missionary efforts even though some of their doctrines do not conform to the norms as held by most Christians. There is evidence enough to come to their doctrinal conclusions, even if we disagree with them.

So is non normative theology enough for mainstream Christianity to label others as non-Christian?
Before we start to answer this, a little history is needed. Both the Latter-Day Saints and Jehovah's Witnesses are Restorationist churches. That means that they were founded on the belief that all the other churches in the world had ceased to hold to the whole correct Christian doctrine, and that a direct revelation from God to the founders caused them to start an entirely new church, and to separate themselves from the 'corrupt' churches that were then in existence (yes, 'corrupt' was the word they used at the time. I'm aware it has a technical meaning that isn't the same as 'dishonest', but it's not very polite even in its technical meaning, which is essentially of a church that has stopped following God). Today both will attempt to convert members of other denominations to their own faith, and insist that people baptized in other Christian denominations are rebaptized to become members of their sects. Cooperation of any kind between Mormons and mainstream churches is extremely rare (compared with cooperation between other denominations), and the non-mainstream sects are at least as assiduous in rejecting it as the mainstream churches. 

So in short, both Mormons and JWs stated for themselves that they are a completely different faith from the mainstream church, and continue to act in that way towards others.

Mormons hold to an additional revelation and additional scriptures, belief in which they consider essential to their own faith, and which are not accepted by other churches and in many cases contradict the tenets of other churches. JWs use a unique Bible translation which they hold to be authoritative and which disagrees in many places with all other translations. Both hold to unique doctrines, particularly about the person of Jesus,  which differ fundamentally from what mainstream churches believe and which they consider to be a non-negotiable part of their faith.

Given this, it is perhaps unsurprising that mainstream churches find it hard to accept churches which do not accept them as valid Christians. However this is by no means a fixed rule. Some denominations do accept Mormons and JWs as a valid form of Christianity.