diff --git a/homeassistant/components/plex/__init__.py b/homeassistant/components/plex/__init__.py
index 386be56340a..97103230288 100644
--- a/homeassistant/components/plex/__init__.py
+++ b/homeassistant/components/plex/__init__.py
@@ -1,5 +1,6 @@
 """Support to embed Plex."""
 import asyncio
+import functools
 import logging
 
 import plexapi.exceptions
@@ -16,7 +17,6 @@ from homeassistant.const import (
     CONF_TOKEN,
     CONF_URL,
     CONF_VERIFY_SSL,
-    EVENT_HOMEASSISTANT_START,
     EVENT_HOMEASSISTANT_STOP,
 )
 from homeassistant.helpers import config_validation as cv
@@ -37,6 +37,7 @@ from .const import (
     DISPATCHERS,
     DOMAIN as PLEX_DOMAIN,
     PLATFORMS,
+    PLATFORMS_COMPLETED,
     PLEX_MEDIA_PLAYER_OPTIONS,
     PLEX_SERVER_CONFIG,
     PLEX_UPDATE_PLATFORMS_SIGNAL,
@@ -72,18 +73,21 @@ CONFIG_SCHEMA = vol.Schema({PLEX_DOMAIN: SERVER_CONFIG_SCHEMA}, extra=vol.ALLOW_
 _LOGGER = logging.getLogger(__package__)
 
 
-def setup(hass, config):
+async def async_setup(hass, config):
     """Set up the Plex component."""
-    hass.data.setdefault(PLEX_DOMAIN, {SERVERS: {}, DISPATCHERS: {}, WEBSOCKETS: {}})
+    hass.data.setdefault(
+        PLEX_DOMAIN,
+        {SERVERS: {}, DISPATCHERS: {}, WEBSOCKETS: {}, PLATFORMS_COMPLETED: {}},
+    )
 
     plex_config = config.get(PLEX_DOMAIN, {})
     if plex_config:
-        _setup_plex(hass, plex_config)
+        _async_setup_plex(hass, plex_config)
 
     return True
 
 
-def _setup_plex(hass, config):
+def _async_setup_plex(hass, config):
     """Pass configuration to a config flow."""
     server_config = dict(config)
     if MP_DOMAIN in server_config:
@@ -141,11 +145,7 @@ async def async_setup_entry(hass, entry):
     )
     server_id = plex_server.machine_identifier
     hass.data[PLEX_DOMAIN][SERVERS][server_id] = plex_server
-
-    for platform in PLATFORMS:
-        hass.async_create_task(
-            hass.config_entries.async_forward_entry_setup(entry, platform)
-        )
+    hass.data[PLEX_DOMAIN][PLATFORMS_COMPLETED][server_id] = set()
 
     entry.add_update_listener(async_options_updated)
 
@@ -167,19 +167,25 @@ async def async_setup_entry(hass, entry):
     )
     hass.data[PLEX_DOMAIN][WEBSOCKETS][server_id] = websocket
 
-    async def async_start_websocket_session(_):
-        await websocket.listen()
+    def start_websocket_session(platform, _):
+        hass.data[PLEX_DOMAIN][PLATFORMS_COMPLETED][server_id].add(platform)
+        if hass.data[PLEX_DOMAIN][PLATFORMS_COMPLETED][server_id] == PLATFORMS:
+            hass.loop.create_task(websocket.listen())
 
     def close_websocket_session(_):
         websocket.close()
 
-    hass.bus.async_listen_once(EVENT_HOMEASSISTANT_START, async_start_websocket_session)
-
     unsub = hass.bus.async_listen_once(
         EVENT_HOMEASSISTANT_STOP, close_websocket_session
     )
     hass.data[PLEX_DOMAIN][DISPATCHERS][server_id].append(unsub)
 
+    for platform in PLATFORMS:
+        task = hass.async_create_task(
+            hass.config_entries.async_forward_entry_setup(entry, platform)
+        )
+        task.add_done_callback(functools.partial(start_websocket_session, platform))
+
     return True
 
 
diff --git a/homeassistant/components/plex/const.py b/homeassistant/components/plex/const.py
index d3c79e60bc4..ad62bade1fd 100644
--- a/homeassistant/components/plex/const.py
+++ b/homeassistant/components/plex/const.py
@@ -9,7 +9,8 @@ DEFAULT_SSL = False
 DEFAULT_VERIFY_SSL = True
 
 DISPATCHERS = "dispatchers"
-PLATFORMS = ["media_player", "sensor"]
+PLATFORMS = frozenset(["media_player", "sensor"])
+PLATFORMS_COMPLETED = "platforms_completed"
 SERVERS = "servers"
 WEBSOCKETS = "websockets"