diff --git a/xilem/src/app.rs b/xilem/src/app.rs
index 9726dedd6..18d594c3a 100644
--- a/xilem/src/app.rs
+++ b/xilem/src/app.rs
@@ -7,7 +7,7 @@ use std::sync::Arc;
 use masonry::core::DefaultProperties;
 use masonry::peniko::{Blob, Color};
 use masonry::theme::{BACKGROUND_COLOR, default_property_set};
-use masonry_winit::app::{EventLoopBuilder, MasonryUserEvent, NewWindow, WindowId};
+use masonry_winit::app::{EventLoopBuilder, MasonryState, MasonryUserEvent, NewWindow, WindowId};
 use tokio::runtime::Runtime as TokioRuntime;
 use winit::error::EventLoopError;
 
@@ -27,6 +27,8 @@ pub struct Xilem<State, Logic> {
     default_base_color: Color,
     // Font data to include in loading.
     fonts: Vec<Blob<u8>>,
+    // Callback invoked once on startup, after windows creation.
+    on_start: Option<Box<dyn FnOnce(&mut MasonryState<'_>)>>,
 }
 
 /// State type used by [`Xilem::new_simple`].
@@ -147,6 +149,7 @@ where
             default_properties: None,
             default_base_color: BACKGROUND_COLOR,
             fonts: Vec::new(),
+            on_start: None,
         }
     }
 
@@ -172,6 +175,12 @@ where
         self
     }
 
+    /// Registers a callback to be called once the application has started
+    pub fn with_on_start(mut self, callback: impl FnOnce(&mut MasonryState<'_>) + 'static) -> Self {
+        self.on_start = Some(Box::new(callback));
+        self
+    }
+
     /// Run app with custom window attributes.
     pub fn run_in(mut self, mut event_loop: EventLoopBuilder) -> Result<(), EventLoopError> {
         let event_loop = event_loop.build()?;
@@ -200,6 +209,7 @@ where
             self.runtime,
             self.default_base_color,
             self.fonts,
+            self.on_start,
         )
     }
 }
diff --git a/xilem/src/driver.rs b/xilem/src/driver.rs
index 9c27dde7e..c52f7f690 100644
--- a/xilem/src/driver.rs
+++ b/xilem/src/driver.rs
@@ -31,6 +31,8 @@ pub struct MasonryDriver<State: 'static, Logic> {
     default_base_color: Color,
     // Fonts which will be registered on startup.
     fonts: Vec<Blob<u8>>,
+    // Optional callback invoked once on startup, after windows creation.
+    start_callback: Option<Box<dyn FnOnce(&mut MasonryState<'_>)>>,
 }
 
 struct Window<State: 'static> {
@@ -54,6 +56,7 @@ where
         runtime: Arc<tokio::runtime::Runtime>,
         default_base_color: Color,
         fonts: Vec<Blob<u8>>,
+        start_callback: Option<Box<dyn FnOnce(&mut MasonryState<'_>)>>,
     ) -> (Self, Vec<NewWindow>) {
         let mut driver = Self {
             state,
@@ -63,6 +66,7 @@ where
             runtime,
             default_base_color,
             fonts,
+            start_callback,
         };
         let windows: Vec<_> = (driver.logic)(&mut driver.state)
             .map(|view| driver.build_window(view))
@@ -367,6 +371,11 @@ where
                 drop(root.register_fonts(font.clone()));
             }
         }
+
+        // Calls callback functions after windows creation.
+        if let Some(cb) = self.start_callback.take() {
+            cb(state);
+        }
     }
 
     fn on_close_requested(&mut self, window_id: WindowId, ctx: &mut DriverCtx<'_, '_>) {