Salesforce: Roll up Activities to Account

In Lightning, activities can be configured as Activity Timeline or Related Lists. As an admin, you can configure this from Setup | User Interface | Record Page Settings | Default Activities View (desktop only).

However, users are also able to personalize the activities view on their own as Activity Timeline or Related Lists from Settings | Display & Layout | Record Page Settings.

Another thing that admin can configure is to enable roll-up activities to a contact's primary account, this means activities (event, task, and email) links to Contact will appear in their direct Account too. To configure this, navigate to Setup | Feature Settings | Sales | Activity Settings, enable roll up activities to a contact's primary account.

Additionally, activities (event, task, and email) in Opportunity and child objects (with Master-Detail relationship to Account only) will appear in Account too, this behaviour is applicable for both views with Activity Timeline or Related Lists. However, Case activities will NOT roll up to Account.

In summary,

* with "roll-up activities to a contact's primary account" is enabled

^ child of Master-Detail relationship

Notes: to enable Contacts links to multiple Accounts, look for Account Settings in the Setup menu, then enable Allow users to relate a contact to multiple accounts.

Activities will not roll up from the child to the parent in a custom object, even with the Master-Detail relationship.

Reference: 

• Enable Roll Up of Activities to a Contact’s Primary Account
• Activities View