diff --git a/gfx/metrics.yaml b/gfx/metrics.yaml
index 419eba8a8ef2..76720b10d195 100644
--- a/gfx/metrics.yaml
+++ b/gfx/metrics.yaml
@@ -89,6 +89,9 @@ wr:
       - gfx-telemetry-alerts@mozilla.com
     expires: never
     telemetry_mirror: WR_RENDERER_TIME
+    send_in_pings:
+      - metrics
+      - pseudo-main
   renderer_time_no_sc:
     type: timing_distribution
     description: >
diff --git a/toolkit/components/glean/metrics_index.py b/toolkit/components/glean/metrics_index.py
index 6028593f9907..92d09d0b1d40 100644
--- a/toolkit/components/glean/metrics_index.py
+++ b/toolkit/components/glean/metrics_index.py
@@ -39,6 +39,7 @@ pings_yamls = [
     "browser/components/newtab/pings.yaml",
     "toolkit/components/glean/pings.yaml",
     "toolkit/components/glean/tests/test_pings.yaml",
+    "toolkit/components/telemetry/pings.yaml",
     "toolkit/mozapps/update/pings.yaml",
 ]
 
diff --git a/toolkit/components/telemetry/app/TelemetryControllerParent.jsm b/toolkit/components/telemetry/app/TelemetryControllerParent.jsm
index a21fa1a623ae..9595ba65be33 100644
--- a/toolkit/components/telemetry/app/TelemetryControllerParent.jsm
+++ b/toolkit/components/telemetry/app/TelemetryControllerParent.jsm
@@ -483,6 +483,20 @@ var Impl = {
     let pingData = this.assemblePing(aType, aPayload, aOptions);
     this._log.trace("submitExternalPing - ping assembled, id: " + pingData.id);
 
+    if (aType == PING_TYPE_MAIN) {
+      try {
+        Glean.legacyTelemetry.profileSubsessionCounter.set(
+          aPayload?.info?.profileSubsessionCounter
+        );
+        GleanPings.pseudoMain.submit(
+          aPayload?.info?.reason?.replaceAll("-", "_")
+        );
+      } catch (e) {
+        this._log.warn("submitExternalPing - Failed to send 'pseudo-main'", e);
+        // Definitely continue, even if things explode.
+      }
+    }
+
     if (aOptions.useEncryption === true) {
       try {
         if (!aOptions.publicKey) {
diff --git a/toolkit/components/telemetry/metrics.yaml b/toolkit/components/telemetry/metrics.yaml
index 4b92b6ff1738..9ad89f25eea2 100644
--- a/toolkit/components/telemetry/metrics.yaml
+++ b/toolkit/components/telemetry/metrics.yaml
@@ -41,3 +41,25 @@ legacy.telemetry:
       - 'events'
       - 'newtab'
       - 'baseline'
+      - 'pseudo-main'
+
+  profile_subsession_counter:
+    type: quantity
+    unit: subsessions
+    description: |
+      The `profileSubsessionCounter` from the Legacy Telemetry "main" ping.
+      This is a value that monotonically increases once for every "main" ping
+      that has been submitted.
+      It is a sequence number by a longer name.
+    bugs:
+      - https://bugzilla.mozilla.org/show_bug.cgi?id=1785251
+    data_reviews:
+      - https://bugzilla.mozilla.org/show_bug.cgi?id=1785251
+    data_sensitivity:
+      - technical
+    notification_emails:
+      - chutten@mozilla.com
+      - glean-team@mozilla.com
+    expires: 113
+    send_in_pings:
+      - 'pseudo-main'
diff --git a/toolkit/components/telemetry/pings.yaml b/toolkit/components/telemetry/pings.yaml
new file mode 100644
index 000000000000..328de45ea32f
--- /dev/null
+++ b/toolkit/components/telemetry/pings.yaml
@@ -0,0 +1,35 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+---
+$schema: moz://mozilla.org/schemas/glean/pings/2-0-0
+
+pseudo-main:
+  description: |
+    A ping designed to be submitted on the same schedule as the Legacy
+    Telemetry "main" ping.
+    See "main" ping documentation for details.
+  include_client_id: true
+  send_if_empty: true
+  reasons:
+    shutdown: |
+      Submitted at shutdown. Likely not uploaded until the following app
+      session. See "main" ping documentation for details.
+    environment_change: |
+      Submitted when something in the "main" ping's Environment changed.
+      See "main" ping documentation for details.
+    aborted_session: |
+      Submitted when the previous session failed to submit a "shutdown"-reason
+      "main" ping.
+      See "main" ping documentation for details.
+    daily: |
+      Submitted around local midnight.
+      See "main" ping documentation for details.
+  bugs:
+    - https://bugzilla.mozilla.org/show_bug.cgi?id=1785251
+  data_reviews:
+    - https://bugzilla.mozilla.org/show_bug.cgi?id=1785251
+  notification_emails:
+    - chutten@mozilla.com
+    - glean-team@mozilla.com
diff --git a/toolkit/components/telemetry/tests/unit/test_TelemetryController.js b/toolkit/components/telemetry/tests/unit/test_TelemetryController.js
index 2e5f53e20579..843fc7fa1c72 100644
--- a/toolkit/components/telemetry/tests/unit/test_TelemetryController.js
+++ b/toolkit/components/telemetry/tests/unit/test_TelemetryController.js
@@ -152,6 +152,11 @@ add_task(async function test_setup() {
   await new Promise(resolve =>
     Telemetry.asyncFetchTelemetryData(wrapWithExceptionHandler(resolve))
   );
+
+  // On Android FOG is set up through head.js.
+  if (AppConstants.platform != "android") {
+    Services.fog.initializeFOG();
+  }
 });
 
 add_task(async function asyncSetup() {
@@ -1217,6 +1222,45 @@ add_task(function test_scalar_filtering() {
   );
 });
 
+add_task(
+  /* After bug 1752139 we should be able to re-enable this. */
+  { skip_if: () => AppConstants.platform == "android" },
+  function test_pseudo_main() {
+    const PING_REASON = "test-reason";
+    const PROFILE_SUBSESSION_COUNTER = 42;
+
+    // Step 0: Clear values.
+    TelemetryController.testReset();
+    Services.fog.testResetFOG();
+
+    // Step 1: Assert no value.
+    Assert.ok(!Glean.legacyTelemetry.profileSubsessionCounter.testGetValue());
+
+    // Step 3: Assert correct value.
+    let pingSubmitted = false;
+    GleanPings.pseudoMain.testBeforeNextSubmit(reason => {
+      pingSubmitted = true;
+      Assert.equal(reason, PING_REASON.replaceAll("-", "_"));
+      Assert.equal(
+        Glean.legacyTelemetry.profileSubsessionCounter.testGetValue(),
+        PROFILE_SUBSESSION_COUNTER
+      );
+    });
+
+    // Step 2: Express behaviour.
+    const payload = {
+      info: {
+        reason: PING_REASON,
+        profileSubsessionCounter: PROFILE_SUBSESSION_COUNTER,
+      },
+    };
+    TelemetryController.submitExternalPing("main", payload, {});
+
+    // Step 3a: Assert we actually ran Step 3.
+    Assert.ok(pingSubmitted, "'pseudo-main' ping was actually submitted");
+  }
+);
+
 add_task(async function stopServer() {
   await PingServer.stop();
 });