Hi,
Could you confirm if what you are looking for is the manager account given the customer ID of your client account? If so, then by logging in the old AdWords UI in your client account, you can go to Account Settings > Account Access. In this page, you can see the "Client managers" section which is the manager/s of the specific client account.
Using the new AdWords UI, you can click the Tools button > Account Access > Managers. You can see on the page the list of manager/s for this client account.
For the AdWords API, you cannot determine the manager account given the client account. It would be the other way around. Given the MCC Id, you can run the
Get Account Hierarchy example (in Java, available to other languages that AdWords API supports) to get all the child sub MCCs and client accounts under that parent MCC. As additional information also, you can run the
CustomerService.getCustomers() without specifying the clientCustomerId in the request header to get all the client accounts which is directly accessible by the authenticated user.
Let me know if you have further clarifications.
Thanks,
Vincent
AdWords API Team