Q: using 2 Past and present tenses in one sentence.

Many times I was confused about using 2 different tenses in one sentence. Is it possible?

For example:
John was my boss, and is my close friend.

I want to talk about my boss (in the past) and he is also my friend (until now). Is this correct?

Yes. The sentence is correct…
…but:
“he is also my friend (until now).”
Not quite. It must be: “he is also my friend”

It is the Present Perfect Tense that refers to the time period ‘up to now’. Present Tense has no time boundaries. So - do you mean:

  1. he was my boss at some time in the past
  2. he was also a friend, and the friendship remained until very recently, and is now over.

If so:

John was my boss, and until recently, was my close friend.
John was my boss, and up to now, has been my close friend.

There may be another way to write this. It depends whether he still is a good friend.

What I tried to say was that

  1. he was my boss at some time in the past
  2. He was also a friend, and the relationship still remain. [Not over yet]

John was my boss once, and is still a close friend.

John was my boss once, and has become a close friend.