How do you manage a high performer who refuses to document their work?

A true high performer is not only an excellent individual contributor, but also someone who shares knowledge, mentors others, and contributes to improving team communication and trust. Someone who is trying to make themselves difficult to replace by hoarding knowledge is not accomplishing any of that. I would start by trying to understand why this person is behaving this way - it may not be malicious. If talking to the person doesn't change anything I would raise it as a risk. You can escalate it to the person's manager, but if the manager doesn't see it as a problem it's not likely to change.

I'm assuming this person is supposed to have a backup. I've been on projects where we've had to wait while one person was out, but there was nobody to replace this person regardless of what was documented.

I would first question whether this is primarily a documentation problem or a performance-system problem.

A person can deliver exceptional individual results while still creating a material dependency for the organization. That does not make them a poor performer, but it does mean that individual output alone may be an incomplete measure of performance when continuity and knowledge transfer matter.

I would also avoid assuming that knowledge kept in someone’s head means they are refusing to share it. I would first understand why it is not transferring and distinguish what can reasonably be documented from knowledge that depends on context, experience or judgment. Documentation is only one mechanism. Pairing, walkthroughs, decision logs, peer reviews and deliberate capability sharing may sometimes work better.

As a manager, I would make the expectation explicit and proportionate to the risk: critical knowledge should not remain an unnecessary single point of failure. I would provide time and practical mechanisms for transferring it and include that contribution in how performance is evaluated.

If those conditions are in place and someone still deliberately refuses to transfer knowledge that is materially necessary for continuity, I would treat that as a performance issue, regardless of how strong their individual results are.

The objective is not to document everything. It is to ensure that exceptional individual capability does not unintentionally become organizational fragility.

When I first say your post, I was thinking more of a project manager that might not be storing the project deliverables in the appropriate location, and the project results could be at risk for audit issues. In this case the job is not done until the paperwork (virtual) is completed.

In your case it sounds like the Digital Marketing Analyst is keeping his tools and knowledge to himself (or herself). As he or she grows in their career, they should be looking more at the team and less at individual results.