Web, OS X, Windows
Todoist stores all your completed tasks in two places - inside projects, where you can see all tasks that were removed from your active task list after completion, and through the Karma window, where you'll see all completed tasks from all projects at once (incl. all completion events for recurring tasks).
(Available to all users...)
To access completed tasks inside a project, click on the circular icon at the bottom of your task list inside a project.
You can un-check any completed task on this list to bring it back to your active task list.
(Available to Premium and Business users...)
To access the full list of all tasks you've completed in any project, click on the Karma score at the top and then select "View all":
You can filter this list by project or label. All tasks on this list are sorted by date and time of completion.
In our mobile apps, you can find the "Completed tasks" option in the menu at the top right while you're inside a project and the full "View all" list in the Karma profile which you can access by tapping on your avatar at the top of the left menu.
You can find the "Completed tasks" option in the menu at the top right while you're inside a project by clicking on the ellipsis icon ( ... ).
You can find the the full list in the Karma profile which you can access by tapping on your avatar at the lower left, selecting "Your Productivity" and then selecting the "XXXX completed tasks" option.