Have been playing around with this answer for a while and I can't get it to work in QGIS 3.4.4:
https://docs.mapbox.com/help/tutorials/mapbox-arcgis-qgis/#add-mapbox-maps-in-arcmap
No help on YouTube or MapBox Tutorials for this scenario.
MapBox keeps saying that works for QGIS 2.x - si that something to do with it?
Tried import via WMS/WMTS (Doesn't connect), via XYZ tiles (raster link shows connection is fine, but doesn't display), via OWS (Doesn't connect).
Choose "Fulcrum" is the answer.
In Mapbox Studio with your style open, hit "Share", "Third Party", and choose "Fulcrum" in the dropdown, and copy the URL.
In QGIS find "XYZ Tiles" in the Browser toolbar, right click, and click "New Connection"
Add the URL from Mapbox Studio to "URL", and type in a "Name". Click OK
Under "XYZ Tiles" in the toolbar, click your new connection to open it
In Mapbox Studio with your style open, hit Share > Third Party > WMTS and copy the URL.
Then in QGIS (tested on 3.6) Layer > Add Layer > Add WMS/WMTS Layer then choose New and paste in the URL from Mapbox into the URL section. Enter a name and hit Ok.
You can use the following XYZ URL by Mapbox as the raster tiles source for QGIS.
https://api.mapbox.com/styles/v1/mapbox/satellite-v9/tiles/256/{z}/{x}/{y}@2x?access_token=<MAPBOX_TOKEN_HERE>
However, you'll need to generate an access token and replace <MAPBOX_TOKEN_HERE> with your token. To generate an access token, you'll need to create a Mapbox account. You can read about access tokens here - https://docs.mapbox.com/help/getting-started/access-tokens/
You can also specify the Mapbox style you want to use. The table here (https://docs.mapbox.com/api/maps/styles/) lists the available styles.
You'll simply need to replace <STYLE_NAME_HERE> in the URL with the style you want to use.
https://api.mapbox.com/styles/v1/mapbox/<STYLE_NAME_HERE >/tiles/256/{z}/{x}/{y}@2x?access_token=<MAPBOX_TOKEN_HERE>
For eg. -
