Add a modified date column to your posts & pages list in WordPress

wp_add_modified_date_column.php

<?php
// Register Modified Date Column for both posts & pages
function modified_column_register( $columns ) {
	$columns['Modified'] = __( 'Modified Date', 'show_modified_date_in_admin_lists' );
	return $columns;
}
add_filter( 'manage_posts_columns', 'modified_column_register' );
add_filter( 'manage_pages_columns', 'modified_column_register' );

function modified_column_display( $column_name, $post_id ) {
	switch ( $column_name ) {
	case 'Modified':
		global $post; 
	       	echo '<p class="mod-date">';
	       	echo '<em>'.get_the_modified_date().' '.get_the_modified_time().'</em><br />';
			echo '<small>' . esc_html__( 'by ', 'show_modified_date_in_admin_lists' ) . '<strong>'.get_the_modified_author().'<strong></small>';
			echo '</p>';
		break; // end all case breaks
	}
}
add_action( 'manage_posts_custom_column', 'modified_column_display', 10, 2 );
add_action( 'manage_pages_custom_column', 'modified_column_display', 10, 2 );

function modified_column_register_sortable( $columns ) {
	$columns['Modified'] = 'modified';
	return $columns;
}
add_filter( 'manage_edit-post_sortable_columns', 'modified_column_register_sortable' );
add_filter( 'manage_edit-page_sortable_columns', 'modified_column_register_sortable' );