diff --git a/newsfragments/2696.feature.rst b/newsfragments/2696.feature.rst
new file mode 100644
index 000000000..560cf3b36
--- /dev/null
+++ b/newsfragments/2696.feature.rst
@@ -0,0 +1,4 @@
+:func:`trio.lowlevel.start_guest_run` now does a bit more setup of the guest run
+before it returns to its caller, so that the caller can immediately make calls to
+:func:`trio.current_time`, :func:`trio.lowlevel.spawn_system_task`,
+:func:`trio.lowlevel.current_trio_token`, etc.
diff --git a/trio/_core/_run.py b/trio/_core/_run.py
index 0b6d32654..50c9cf258 100644
--- a/trio/_core/_run.py
+++ b/trio/_core/_run.py
@@ -2118,6 +2118,16 @@ def start_guest_run(
     the host loop and then immediately starts the guest run, and then shuts
     down the host when the guest run completes.
 
+    Once :func:`start_guest_run` returns successfully, the guest run
+    has been set up enough that you can invoke sync-colored Trio
+    functions such as :func:`current_time`, :func:`spawn_system_task`,
+    and :func:`current_trio_token`. If a `TrioInternalError` occurs
+    during this early setup of the guest run, it will be raised out of
+    :func:`start_guest_run`.  All other errors, including all errors
+    raised by the *async_fn*, will be delivered to your
+    *done_callback* at some point after :func:`start_guest_run` returns
+    successfully.
+
     Args:
 
       run_sync_soon_threadsafe: An arbitrary callable, which will be passed a
@@ -2178,6 +2188,39 @@ def my_done_callback(run_outcome):
             host_uses_signal_set_wakeup_fd=host_uses_signal_set_wakeup_fd,
         ),
     )
+
+    # Run a few ticks of the guest run synchronously, so that by the
+    # time we return, the system nursery exists and callers can use
+    # spawn_system_task. We don't actually run any user code during
+    # this time, so it shouldn't be possible to get an exception here,
+    # except for a TrioInternalError.
+    next_send = None
+    for tick in range(5):  # expected need is 2 iterations + leave some wiggle room
+        if runner.system_nursery is not None:
+            # We're initialized enough to switch to async guest ticks
+            break
+        try:
+            timeout = guest_state.unrolled_run_gen.send(next_send)
+        except StopIteration:  # pragma: no cover
+            raise TrioInternalError(
+                "Guest runner exited before system nursery was initialized"
+            )
+        if timeout != 0:  # pragma: no cover
+            guest_state.unrolled_run_gen.throw(
+                TrioInternalError(
+                    "Guest runner blocked before system nursery was initialized"
+                )
+            )
+        next_send = ()
+    else:  # pragma: no cover
+        guest_state.unrolled_run_gen.throw(
+            TrioInternalError(
+                "Guest runner yielded too many times before "
+                "system nursery was initialized"
+            )
+        )
+
+    guest_state.unrolled_run_next_send = Value(next_send)
     run_sync_soon_not_threadsafe(guest_state.guest_tick)
 
 
diff --git a/trio/_core/_tests/test_guest_mode.py b/trio/_core/_tests/test_guest_mode.py
index 7b004cf04..7aef3e437 100644
--- a/trio/_core/_tests/test_guest_mode.py
+++ b/trio/_core/_tests/test_guest_mode.py
@@ -26,7 +26,7 @@
 #   our main
 # - final result is returned
 # - any unhandled exceptions cause an immediate crash
-def trivial_guest_run(trio_fn, **start_guest_run_kwargs):
+def trivial_guest_run(trio_fn, *, in_host_after_start=None, **start_guest_run_kwargs):
     todo = queue.Queue()
 
     host_thread = threading.current_thread()
@@ -58,6 +58,8 @@ def done_callback(outcome):
         done_callback=done_callback,
         **start_guest_run_kwargs,
     )
+    if in_host_after_start is not None:
+        in_host_after_start()
 
     try:
         while True:
@@ -109,6 +111,48 @@ async def do_receive():
     trivial_guest_run(trio_main)
 
 
+def test_guest_is_initialized_when_start_returns():
+    trio_token = None
+    record = []
+
+    async def trio_main(in_host):
+        record.append("main task ran")
+        await trio.sleep(0)
+        assert trio.lowlevel.current_trio_token() is trio_token
+        return "ok"
+
+    def after_start():
+        # We should get control back before the main task executes any code
+        assert record == []
+
+        nonlocal trio_token
+        trio_token = trio.lowlevel.current_trio_token()
+        trio_token.run_sync_soon(record.append, "run_sync_soon cb ran")
+
+        @trio.lowlevel.spawn_system_task
+        async def early_task():
+            record.append("system task ran")
+            await trio.sleep(0)
+
+    res = trivial_guest_run(trio_main, in_host_after_start=after_start)
+    assert res == "ok"
+    assert set(record) == {"system task ran", "main task ran", "run_sync_soon cb ran"}
+
+    # Errors during initialization (which can only be TrioInternalErrors)
+    # are raised out of start_guest_run, not out of the done_callback
+    with pytest.raises(trio.TrioInternalError):
+        class BadClock:
+            def start_clock(self):
+                raise ValueError("whoops")
+
+        def after_start_never_runs():  # pragma: no cover
+            pytest.fail("shouldn't get here")
+
+        trivial_guest_run(
+            trio_main, clock=BadClock(), in_host_after_start=after_start_never_runs
+        )
+
+
 def test_host_can_directly_wake_trio_task():
     async def trio_main(in_host):
         ev = trio.Event()