Activity history on a Lead when it is converted

You can assign the owner of the records, and schedule a follow-up Task. When you assign a new owner, open Tasks assigned to the User converting the Lead will be reassigned to the new owner. Tasks assigned to other Users will not be reassigned. For Tasks in Activity History, the Assigned user will be retained, as long as the user has access to the related records, otherwise they will also be assigned to the new owner. Events assigned to the User converting the Lead will be reassigned to the new owner, regardless of whether they are Open Activities or listed in Activity History. 

If you have custom Lead fields, that information can be inserted into custom Account, Contact, or Opportunity fields as well. Prior to converting Leads, your administrator must set this up: Map Custom Lead Fields for Lead Conversion

Converted leads can't be viewed, although they appear in lead reports.

Salesforce updates the Last Modified Date and Last Modified By system fields on converted Leads when picklist values included on converted Leads are changed.

As part of the Lead conversion process, the Last Modified Date, and Last Modified By system fields are also updated on the related open and closed activities that exist for the Lead that is converted.  The reason this occurs, is since when the Lead conversion is done, the related activities have to have the values in the "Name" and "Related To" values set to the Contact (Name) and Opportunity (if an Opportunity is created on Lead convert) as part of the lead conversion process (Related To). 

All notes and attachments from the lead are converted and attached to the new account and contact. All open activities and activity history from the lead are converted and attached to the new account, contact, and opportunity. The converted lead record can no longer be viewed, although it does contribute data to reports.
 

See Also:

Considerations for Converting Leads
Leads: Update Details for Converted Leads in Salesforce Classic
View converted Leads