Assuming I have created an email in hushmail through of tor, and if I use it as my official email in a site such as Github. Will it throw my anonymity away as I use github from a regular browser?
Yes. If you use an account on Tor and also use the same account not on Tor, your ownership of the account can be traced via the use not on Tor. If you link two accounts (e.g. by listing an email from Tor on a non-Tor account), someone could potentially find that link and then trace the non-Tor account. This is one of many reasons "use Tor" isn't enough to maintain anonymity; you need to be very careful about what information you're leaving about yourself and where.
To a large extent, yes, you will lose your anonymity. Github (and anyone monitoring Github) will be able to connect your email address to your IP address.
A key element of anonymity (or more technically, pseudonymity) is keeping your separate identities separate. Your Hushmail account accessed over Tor is one identity, while everything you do through direct Internet connections constitutes a second identity. By using your Hushmail account for a direct-connection service, you've tied those two identities together.