my mastiff is in her 1st season at the moment.when is best to mate her

On her third season at the earliest.

I would say after 2 year old before that you cannot get a full picture of what your girl is going to be, if she conforms to the breed standard or as near as possible no dog is perfect, it would also give you a rough idea if she was going to inherit any health problems. Mastiff breeds are usually slow maturers I know with out Tibetan mastiff's and Dogue de Bordeaux that they are not fully mature untill they are between 3-4 years old, she would need hip scoreing if you have not already had it done I always get elbows x-rayed at the same time, but your girls breeder should be able to advise you