Currently it is not standard functionality to allow the Comment section of the Tasks to show in the Activity History or the Open Activity.

The workaround to allow the comments to show on the related list is:

  • Setup Custom text field of length upto 255 characters and then update this text field with text added in comment section with workflow rule-:

  1. Setup> Customize> Activities> Activity Custom Fields
  2. Click New
  3. Select the Text field
  4. Clicked next
  5. Naming the field (Notes), click Next
  6. Then save.   
  7. To create the workflow you would go to:
  8. Setup> Create> Workflow & Approval
  9. Clicked Workflow Rules
  10. Click the New Rule button
  11. Select the Task Object, click next
  12. Name the workflow rule
  13. Set to Evaluate the rule when a record is created, and every time it’s edited.
  14. Set the criteria to: 
  15.                                       Field: Comments
  16.                                       Operator: Not Equal To
  17. ​                                      Value: Null
  18. Then click Save and Next
  19. In immediate actions
  20. Click the drop down Add Workflow Action and select Add Field update
  21. Name the field update
  22. Click the field drop down and select the custom field (Notes)
  23. Select to update with a formula and then added the formula IF(LEN(Description)>255,LEFT(Description,252)&"...",Description),
  24. Click Save, and click Done
  25. Click Activate
  26. Then go to the Page Layout of the Object that you require the field to show on the related list, then:
  27. Scroll down to the Activity History or Open Activity Related List.
  28. Click on the Wrench.
  29. Find the field (Notes) in Available fields.
  30. Select the Field and click Add.
  31. Once added click the up and down arrow to position the field on the related list (Optional).
  32. Click OK, then the field will be visible in the related list
  33. Note:
  34. Task ‘Comments’ field is a Long Text Area field which will have more than 255 characters, the formula ‘Note’ field will only show first 255 characters from the standard ‘Comment’ field.