You can programmatically create posts and pages using wp_insert_post
or insert content using WXR files via WordPress’s import feature.
To use wp_insert_post
see the documentation here: http://codex.wordpress.org/Function_Reference/wp_insert_post
A simple example:
$my_post = array(
'post_title' => 'hello',
'post_content' => 'This is my post.',
'post_status' => 'publish',
'post_author' => 1,
'post_category' => array(1),
'post_type' => 'page'
);
// Insert the post into the database
wp_insert_post( $my_post );
BUT you are probably better off using WordPress’s import functionality. WordPress uses WXR files which stands for WordPress Extended Rss ( if you know XML you will see it looks similar).
So it would be better for you to parse you mongo content into a WXR file and then just import it into WordPress.
A detailed write up on WXR: http://devtidbits.com/2011/03/16/the-wordpress-extended-rss-wxr-exportimport-xml-document-format-decoded-and-explained/
Also you can just add some dummy content to WP and export it and view the WXR file to see what format you should follow.