Company I work at asked me to write a job description for my role. Is this common thing? First time I've heard it

Writing your own job description is somewhat common, though I've heard it more in the form of a "vision" exercise where you imagine what your ideal job would be.

The scenario you're talking about has a constraint that your job description should fit within the needs of the business, and you're formalizing what you were doing at the startup. Pretty cool, and an opportunity to re-invent yourself slightly if you'd like!

The oddity of your scenario is that it sounds like this will dictate your compensation 💸 that adds a lot of pressure! Before you "submit" your job description, you should be very deliberate and do the following:

• Read through whatever documentation exists about expectations for various levels at the company. If you borrow the verbiage of the next level, there's a better chance you'll get higher compensation.
• Share a draft of the job description with your manager. One way of framing it is, "I'd like the job description to not only capture what I've been doing, but also how I expect the role to grow and evolve over the coming months."

You can also use this as an opportunity to reflect and think about your job today, and what parts you like or don't like, e.g. do you have adequate work/life balance, how much IC work are you doing vs "glue work"? I replied to an interesting discussion about that here:

How to Balance Responsibilities: Prioritizing Personal Work vs. 'Glue Work' in a New Team Environment.