diff --git a/mercurial/dispatch.py b/mercurial/dispatch.py
--- a/mercurial/dispatch.py
+++ b/mercurial/dispatch.py
@@ -828,6 +828,7 @@ def _dispatch(req):
             color.setup(ui_)
 
         if util.parsebool(options['pager']):
+            # ui.pager() expects 'internal-always-' prefix in this case
             ui.pager('internal-always-' + cmd)
         elif options['pager'] != 'auto':
             ui.disablepager()
diff --git a/mercurial/ui.py b/mercurial/ui.py
--- a/mercurial/ui.py
+++ b/mercurial/ui.py
@@ -945,8 +945,14 @@ class ui(object):
                    not "history, "summary" not "summ", etc.
         """
         if (self._disablepager
-            or self.pageractive
-            or command in self.configlist('pager', 'ignore')
+            or self.pageractive):
+            # how pager should do is already determined
+            return
+
+        if not command.startswith('internal-always-') and (
+            # explicit --pager=on (= 'internal-always-' prefix) should
+            # take precedence over disabling factors below
+            command in self.configlist('pager', 'ignore')
             or not self.configbool('ui', 'paginate')
             or not self.configbool('pager', 'attend-' + command, True)
             # TODO: if we want to allow HGPLAINEXCEPT=pager,
diff --git a/tests/test-pager.t b/tests/test-pager.t
--- a/tests/test-pager.t
+++ b/tests/test-pager.t
@@ -80,6 +80,34 @@ We can control the pager from the config
   paged! 'summary:     modify a 10\n'
   paged! '\n'
 
+explicit --pager=on should take precedence over other configurations
+(issue5580)
+
+  $ cat >> $HGRCPATH <<EOF
+  > [ui]
+  > paginate = false
+  > EOF
+  $ hg log --limit 1 --pager=on
+  paged! 'changeset:   10:46106edeeb38\n'
+  paged! 'tag:         tip\n'
+  paged! 'user:        test\n'
+  paged! 'date:        Thu Jan 01 00:00:00 1970 +0000\n'
+  paged! 'summary:     modify a 10\n'
+  paged! '\n'
+
+  $ cat >> $HGRCPATH <<EOF
+  > [ui]
+  > # true is default value of ui.paginate
+  > paginate = true
+  > EOF
+  $ hg log --limit 1 --pager=off
+  changeset:   10:46106edeeb38
+  tag:         tip
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     modify a 10
+  
+
 We can enable the pager on id:
 
 BROKEN: should be paged