diff --git a/core/core-ui.el b/core/core-ui.el
index d81625c91..848bb03f2 100644
--- a/core/core-ui.el
+++ b/core/core-ui.el
@@ -634,6 +634,15 @@ behavior). Do not set this directly, this is let-bound in `doom-init-theme-h'.")
       result)))
 
 (when! (not EMACS27+)
+  ;; DEPRECATED `doom--load-theme-a' handles this for us after the theme is
+  ;;            loaded, but this only works on Emacs 27+. Disabling old themes
+  ;;            must be done *before* the theme is loaded in Emacs 26.
+  (defadvice! doom--disable-previous-themes-a (theme &rest no-confirm no-enable)
+    "Disable other themes when loading a new one."
+    :before #'load-theme
+    (unless no-enable
+      (mapc #'disable-theme (remq theme custom-enabled-themes))))
+
   ;; DEPRECATED Not needed in Emacs 27
   (defadvice! doom--prefer-compiled-theme-a (orig-fn &rest args)
     "Have `load-theme' prioritize the byte-compiled theme.