When writing about your deceased father's occupation, you can still use the past tense to indicate that he was engaged in that occupation during his lifetime. Here's how you can write it:
"Before his passing, my father was a [occupation], and he worked in that field for many years."
For example, if your father was an engineer, you would write:
"Before his passing, my father was an engineer, and he worked in that field for many years."
Using the past tense acknowledges that your father's occupation was a significant part of his life while he was alive. It's a respectful way to refer to his career and honor his memory.