You can request a letter of recommendation from a former supervisor, and it can be just as valuable as one from a current supervisor. A former supervisor can provide a unique perspective on your past work and accomplishments and speak to your growth and development over time. However, ensuring that your former supervisor is still reachable and willing to write a letter for you is vital. If you haven't been in contact for a while, please reach out beforehand to catch up and ask if they are willing to write a letter of recommendation.