You can get all pull requests (closed, opened, merged) through the variable state.
Just set state=all in the GET query, like this->
https://api.github.com/repos/:owner/:repo/pulls?state=all
For more info: check the Parameters table at https://developer.github.com/v3/pulls/#list-pull-requests
Edit: As per Tomáš Votruba’s comment:
the default value for, “per_page=30”. The maximum is per_page=100. To get more than 100 results, you need to call it multiple itmes: “&page=1”, “&page=2″…